This has got a mix of canon from the books and from what we've seen in the trailers. I made some stuff up concerning various differences between the movie-info and book-info, like how Nightlight is gone...anyway, enjoy!
There was a tell-tale trail of slush leading from the back door, through the kitchen and on a few rounds through the halls, finishing out in the main observatory. That, and the various disgruntled elves who complained of it to North made it obvious that Jack Frost had decided to pay a visit.
The boy was discovered at last, sprawled in North's favorite leather arm-chair, the one with the view of the Globe which displayed the clouds of dreams drifting around the world. Jack eyed it lazily, the way other children watch cartoons, and scooped cookie dough out of what could only be called a vat into his mouth by the fistful.
The silent appearance of North at his shoulder made Jack jump, and he exclaimed through a mouthful of dough, "The elves said I could help myself! They said."
"I'm sure they did," North rumbled, trying to appear stern. The truth was that North often sat like that, in the exact same attitude, with the exact same vat containing the exact same amount of cookie dough, so he couldn't in all fairness be stern about anything.
Jack began to wipe his fingers off on the arm of the chair, but caught North's look just in time and wiped them off on his sweater instead. "It's good. Double-chocolate chip."
"With peanut butter?"
"Yeah, just a bit."
North smiled. "That's the best kind. Not enough children know about the peanut butter."
"Mm," Jack settled back in the chair, trying to appear nonchalant. He gestured at the Globe. "I like it."
"It's nice when the dreams stay good, and the skies stay clear and golden," North agreed. "But sometimes there are storms. That's where we come in."
Jack's frown travelled around his face in little patterns as he tried to figure out what to say to that.
"But the clear nights, where everything just shines," North continued. "That's what makes all the work worth it."
"It's pretty," Jack replied eventually, pushing himself out of the chair. "Well, I should be-"
"Did you want to spend the night?"
Jack shot North his particular look, that glance like a startled puppy afraid of being accused for the lamp that mysteriously ended up in pieces on the floor. "Huh?"
North shrugged. "I thought maybe you came in because you wanted to spend the night. There is plenty of spare room here, you know. You could stay."
But Jack continued to eye the door as if it was his only hope, so North shrugged and tried not to be disappointed. "Well, feel free to visit whenever you like. My door is open."
Jack Frost was, contrary to all expectation, a very neat house guest. In the morning, all trace of slush was gone, and the cookie-dough vat had been scrubbed out and dried. The elves reluctantly denied responsibility for cleaning up Jack's trail.
"That's good," Bunnymund sniffed. "That's exactly what you need. Holes in your security system, and a nighttime invader coming and going as he pleases."
"He is an ally now," North reminded his friend firmly. "And young." He sighed. "I can't force him, but I wish he would just stay where I can keep an eye on him. I would worry less."
"You think that now," Bunnymund mused. "But if he stayed, then you would want to know where he was at all times. You would hover around him like a mother hen and never leave him alone. Jack values his privacy."
"I have no stipulations to put on him," North protested. "He could come and go as he pleased."
"Yes, but could he really?" Bunnymund raised an eyebrow at North. "I know you leave children be most of the year, but it's different to have one living in the house."
North rolled his eyes. "It doesn't matter. He's never going to say yes."
A few nights later, the telltale signs had returned. This time North didn't need to bother following the meandering trail around the cave. He headed straight for his armchair, where he could see Jack's skinny ankles dangling off the side.
"No cookie dough tonight?" North rumbled as he turned the chair around to face him.
Jack lay, limp, in the chair, face flushed red. He croaked, "No, thanks."
North knelt next to the boy and felt Jack's temperature. His skin was room-temperature to the touch; in Jack's terms, a blistering fever.
Jack shrugged feebly. "I just came to watch the lights."
When Bunnymund finally arrived, North had worked himself up into a frenzy.
"I don't know what to do!" North paced up and down the observatory. "I don't know if I should be drowning a cold, or starving a flu! I don't know if I should be trying to warm him up or cool him down!"
"You could always throw him in a snow bank," Bunnymund suggested. "He would probably be more comfortable."
North glared at him. "Don't you have any medicine or something?"
Bunnymund shrugged. "Besides chocolate? I don't think he needs more candy after all that cookie dough. It's a stomach ache. Children get those when they eat too many sweets."
"He didn't have any cookie dough-"
"How do you know?"
"Because he told me!"
"Well, that's one mystery solved. He lied," Bunnymund kicked his paws out lazily. "He'll sleep it off tonight, and come back for more tomorrow."
"Rabbit," North growled. "You were very angry with me once for not taking this sort of thing seriously."
"No, I was angry with you for being a hypocrite," Bunnymund replied smoothly. "Getting angry with another child for making a bid for attention-"
"Aha!" North cried, pointing a wild finger. "So you admit he was just doing it for attention!"
"A bid for gratitude," said Bunnymund. "And Nightlight was absolutely nothing like Jack."
He winced, regretting his comment, as sorrow crossed North's face.
"I don't think of Jack as Nightlight's replacement," North mumbled reproachfully. "It's very different. That's why I'm trying to do better this time."
After a few moments, Bunnymund said, "Well, it's off-season for me." Bunnymund winced as pure joy permeated North's face; he already regretted what he was about to offer. "I could always-"
"Sit up with him? Thank you, my friend!"
North bounded away, too joyous at his friend's sudden generosity, and too stressed with the approach of the holidays, to think twice. Not that this was particularly unusual.
With a groan, Bunnymund pushed himself up and padded to the guestroom North had organized for Jack. It was, suspiciously enough, already set up with a bed, a desk, a closet, and a large window, the sort of thing an outdoorsy-type young boy would find comforting. North had denied preparing any sort of permanent living accommodation for Jack before getting the boy's consent.
Bunnymund prepared to snort derisively as he came through the door, but ended up choking on it when he saw the Tooth Fairy, snuggled up comfortably with Jack in bed.
Granted, she was lying on top of the covers; even Tooth generally had a sense of decency even if her outlandish instincts prohibited her from having common sense. She had wrestled Jack under her arm and onto her lap, and he lay there, arms wrapped around her waist, head resting on her belly. She rubbed his back with one hand and ruffled his hair with the other. The pair looked offensively comfortable.
Tooth twittered by way of a greeting at Bunnymund.
"What are you doing here?"
"Obviously I'm visiting my favorite little sweet tooth!" Tooth squeaked, mussing Jack's hair for emphasis. "Isn't he cute when he's sleeping?"
"Children generally are cutest when they are asleep," said Bunnymund darkly, "Because it prohibits them from causing trouble."
"I got North's message," Tooth continued blithely, as though Bunnymund had not spoken. This was her usual response to his sarcasm. "So I came by to see how I could help."
"North only sent a message to me."
Tooth shrugged. "He sent out a general query for ideas. Poor thing was panicking. He's such a fuddy-duddy." She smirked. "And you are so possessive."
Bunnymund snorted. "He always calls me when he needs help ,that's all." More seriously, he added, "We were wondering about Jack over-heating. Is he going to be alright, wrapped up in all those blankets?"
"We can always get rid of the blankets," Tooth shrugged. "The best cure for everything is sitting with Mummy and having a backrub. So that's what I'm doing."
"No teeth tonight?"
Tooth bristled at that, but replied in an even tone of voice, "There are always teeth. There are a lot tonight, in fact. I'm spread out all over, so it's actually easier for me to sit down and just focus on one thing, otherwise I'll get dizzy."
The Tooth Fairy doled out money for teeth-and memories-by making divisions within herself and flying all over the world. Unlike the other Guardians, she really could be everywhere at the same time. It came at the cost, however, of being widely considered a scatter-brained mess. A competent warrior and a dangerous adversary, of course, but a mess all the same.
Very sweetly, she added, "And of course, it's off-season for you, so you have plenty of time to help me with Jack. I'll need someone to do a bit of fetching and carrying."
Bunnymund sighed, and pulled up a chair next to the bed. "I'm sorry. I just don't like seeing North get so worked up. He has enough to do."
Tooth smiled lazily at him. "Well, we all need to take care of each other." She chuckled. "You and I bicker like spouses."
Bunnymund smiled. "Yeah…" He suddenly sat bolt upright. "Not that I think of you that way."
"No, no," Tooth nodded emphatically. "I meant it as a figure of speech."
"Good. Yes, well…good."
"I am what?"
"You must know best what to do in these situations," Tooth rattled on. "Sitting up with sick children. All those bunnies? Excuse me, Pookas? I think of them all as bunnies. Not that I think of them. You. Bunnies. Um."
"Of course, it was a while ago, but I can just imagine a warren full of-not that I assume you all live in warrens, I'm sure your people were very sophisticated-"
Tooth had apparently run out of gabble, so smiled weakly instead. "Bunnies. Children. You had some at one point, right?"
Bunnymund stared at her.
"North mentioned it," She continued slowly. "He said he was calling you for advice, because you probably had lots of experience with sick children. Being a father and all."
Eventually, Bunnymund heard himself say, "Well, physiologically speaking, Jack is probably quite different than most young rabbits."
"Yes, of course," Tooth agreed quickly. "Silly of me. Never mind."
"Not at all."
"Always talking ahead of myself. Hard to focus."
"I mean, you never talk about it, and I've just got no sense about these things-"
"Tooth, please just stop."
Bunnymund was genuinely furious for a moment. And then he realized that Tooth looked as though she might cry.
"I don't talk about my family very often," he said finally.
"You don't ever," she corrected him quietly.
"It was a very long time ago." Bunnymund frowned, concentrating. "There are other things to do."
"Other children to take care of?" Tooth suggested, but the rabbit wasn't listening.
"It's very easy," Bunnymund continued in a sharp tone of voice, "to drop in once a year on a child and make their day. Being a parent is very different. We forget that. Having a child all the year round is a commitment." His frowned deepened for a moment, and then he let it go. "North doesn't understand that at all."
Tooth squirmed uneasily. "I wouldn't quite call Jack a child -"
Bunnymund laughed humorlessly. "Of course you would. He needs taking care of. Petting and molly-coddling. The cure for everything, as you say, is sitting up with Mummy and getting a back-rub."
Tooth licked her lips and clicked her teeth a few times before saying, "Yes, but I never said that mummies and backrubs are just for children."
"Meaning," she said slowly, "that everybody needs taking care of once in a while."
Before Bunnymund could respond, Jack kicked at his blankets and moaned, "Too hot."
"Oh, I'm sorry sweetie!" Tooth chirped. "Bunnymund, get rid of all these awful blankets, will you?"
Bunnymund gritted his teeth and complied while Tooth stroked Jack's forehead and chattered amiably.
"North said you took a bad turn. How are you feeling, love?"
"Kinda warm. And tired." Jack looked up at Tooth, frowning. "What are you doing here?"
"Silly! I came to see you!"
Jack's eyes widened, traces of the guilty-puppy expression Bunnymund hated so much coming into view. "You came to see me?"
Above him, where Jack couldn't see, Tooth shot Bunnymund a heart-melting look. "Yes, of course, dear! I wanted to make sure you were alright."
Jack didn't seem to know how to take this. "No one's ever visited me before," He mumbled.
"It generally comes with the having of a permanent address," Bunnymund said, much more sharply than he intended. He focused on folding the blankets so he would not have to endure Tooth's glare.
The comment seemed to fly right over Jack's head. "Yeah," He said drowsily. "I should look into that."
Tooth continued to fuss over him as Bunnymund dragged the blankets away.
He deposited the blankets as neatly as he felt able in the nearest corridor, and went to North's workshop.
North was tinkering with some sort of train-always trains, Bunnymund thought-when Bunnymund found him. The rabbit had to clear his throat six times before North noticed he was there.
"How is he?"
"Tooth is sitting with him," Bunnymund said shortly, "And doing a much better job of it than I ever could, I'm sure. We need to talk."
North gestured at the train set behind him, and said, "I would love to, but-"
"No. No buts. If you want Jack to live here with you, we need to get some things straight. First of all, you're going to need to be able to drop everything at a moment's notice. Children need immediate attention."
North laughed. "I wouldn't call Jack a child-"
"Yes, he is," said Bunnymund. "North, our first mistake with Nightlight was assuming that he wasn't like other children, and look what happened. If you want to do better, start with this simple fact: Jack is a child. Or at the very least, he needs taking care of the way children do."
North recognized that Bunnymund was working himself into a rare stream of consciousness, and sat down to listen.
"You have to make time for children. Eventually they'll learn a bit of patience, but they need to know that in your list of priorities, they rank high. They need a good-sized leash. They need to know that you mean what you say. If you're going to make threats about privileges, you better be willing to follow through. If they ever figure out that you're bluffing, you're finished, they'll never listen to you again. And you need to know when to let go and give them free reign of it. There," Bunnymund said, panting. "That's everything I know about parenting. I wish you had just come out and asked me."
North smiled gently at his friend. "You never talk about your family. I did not wish to be rude."
Bunnymund sagged against the wall. "I don't think I was a particularly good father. But I learned a lot from it. It might be good for you or something. I honestly couldn't say." He took a deep breath. "You're busy. I'll go."
He nearly made it out of the hall before he remembered. Bunnymund poked his head back into the workshop to add," You're going to be tired, and frustrated, and torn up over little things, pretty much all the time. Just don't forget that sometimes you need taking care of, too."
He left before North could respond.
And promptly collided with Tooth.
She giggled as they lay in a tangle on the floor. "I was just coming to say that Jack is asking for something to drink. Is there hot chocolate?"
"I'll get it," Bunnymund flexed a long leg tentatively, and recoiled when he felt it touch Tooth's shoulder. "Sorry."
"Maybe you could go sit with him for a minute," said Tooth as they picked themselves up.
"I don't think that would be a good idea," Bunnymund said firmly.
"But you're so good with children-"
"Jack is not a replacement for my children!" Bunnymund took a few deep breaths to calm himself down. "I thought I was ready for something like this when Nightlight came along. And I wasn't. So I'm not falling for that again. If I indulge him once…and besides, North wants him to stay. I've just spoken with him. I can help, of course, but this is his problem, not mine."
Tooth stared at him for a moment. "Jack looks up to you," she said finally. "You might think about that."
"He picked the wrong one of us to-"
"I'm going to make you some hot chocolate as well!" Tooth chirped emphatically as she sped away. "Go sit with him until I get back!"
Bunnymund glared ferociously at her retreating back. "The kitchen is the other way!" He shouted after her.
He fumed the whole walk back to the guest-well, Jack's-room. Tooth was scatter-brained enough to think that she would reach the kitchen by heading on in one direction forever, the same way that circling the globe worked. But she had probably gone the wrong way on purpose.
Jack was sitting up in bed, looking like a kicked puppy as usual. He slouched even more when he saw Bunnymund at the door.
"Tooth is getting some hot chocolate," Bunnymund said finally, sitting down in the chair next to the bed.
"I wasn't faking it," Jack mumbled. "I really did feel sick."
"Too much cookie dough?"
"Well," Bunnymund sighed, "Maybe you learned something then." It was a line he had used with his children, a long time ago. It usually seemed to work.
Jack nodded, keeping his eyes down.
"I'm glad you thought to come to North," Bunnymund added after a moment. "You can always come to one of us for help. That's what we're here for. Y-you know that, right?"
Jack looked up at him for a moment. "Yeah. Yeah, okay."
Tooth didn't return with the hot chocolate for a long time. When she did, she had three mugs.
"Extras for yourself?" Bunnymund asked drily.
As usual, she didn't rise to it. "It's for you," She said. "Help me with these, it's hard to balance a tray."
North found them there later, all asleep and with empty mugs of hot chocolate. Bunnymund had nodded off in his chair, while Tooth and Jack snuggled together on the bed. North fetched the blankets Bunnymund had left out in the hall and covered them each with one.
As North turned out the light and closed the door, he turned back to the room and announced softly, "Come over any time you like. I like having guests."