Any warnings in case you are unaware: this story features infantilism, which means grown adult/s acting like infants. Spock is the one who is the "infant" in this scenario, and Bones is his "daddy". Because of this, Spock does and should seem highly out of character, and has no emotional boundaries, and Bones bathes and diapers him. The scene is purely non-sexual. If this criteria does not meet your expectations, it would be better to leave now than get your hopes up.
Leonard had gotten him the crayons on a whim, after dropping his overnight bag in their room. Spock had insisted on having a conversation with Jim about the negotiations going on in the Capitol between them and the native species. He'd headed out into the city to work off some boredom, and had come across a few stores that sold real paper and real crayons. It was hard to come by real paper in this day and age, even harder on a starship in the middle of deep space, and crayons were nearly non-existent. It had been a surprise that the planet they were orbiting while the negotiations were established still used paper and still melted colored wax for children's use, but a pleasant one. They never had paper on Vulcan, and they certainly never had crayons, that he was sure of. Combine the fact that Spock had never had the opportunity to just scribble away on a scrap of paper to his heart's content to the fact that they were now able to attain actual paper and crayons, and it was one of the most fortunate things that'd happened to him off-ship.
The two of them had been given a room at the palace in the heart of the capitol city, where they were supposed to be staying overnight so Spock and Jim could get up at the crack of dawn and keep up with the conferences between the high rulers or whatever it was they called themselves. If Leonard was honest with himself, he hadn't been paying that much attention. His input was hardly needed; he was only down there in case Jim had an allergic reaction or something else just as ridiculously possible. He'd also had his attention taken up by making sure Spock was comfortable. Spock was his responsibility, First Officer on the Federation's flagship or not, and protecting him and devoting his time and affection to him was just as important as any old planet in this galaxy or the next.
As it was, he knew Spock had been uncomfortable during the initial meet-n-greet with the diplomats and ambassadors that had met them at the energizing platform, and also knew that he'd been uncomfortable during the hours-long negotiations that had followed. Leonard had been by his side through the thick of it, feeling him shift minutely in his seat, listening to his small sighs that escaped him, seeing the way he shivered almost imperceptibly the longer it took for the negotiations to close. When even Jim had started getting tired of the constant talking, he'd asked the diplomats if it'd be alright for he and his crew to stay on-planet overnight so they could wrap it up early the next day. Of course they'd agreed, hence Leonard's current situation.
He'd set the stack of paper he'd bought as well as the pack of crayons on the bedside table (and how many different colors could you make with a bunch of wax sticks?) moments before Spock entered the room, and Leonard could immediately see that he was wilting. His Vulcan mask was gradually breaking down, shoulders slumping, features becoming less stoic the longer he spent inside this room, alone with Leonard. It made him smile. "Hey, baby boy," he said, keeping his voice low and smooth, and Spock relaxed even further, walking to him. It was more of a toddle than a walk, which meant that he was feeling safer and safer the more he talked and sinking into the scene. He held open his arms and beckoned him closer, settling into his own role as easily as he breathed when Spock immediately curled around him, lifting his arms with clumsy eagerness around his neck and nuzzling his cold nose into his pulse point. He was humming with contentment, but he wasn't quite all the way there yet.
That'd be an easy fix.
He carted him to the connected bathroom, murmuring terms of endearment and sweet nothings to his baby all the way, until Spock was flushed with happiness and squirming with delight at the praise. His eyes shone with love, pure and unadulterated, but he was shivering outright now, the planet's atmospheric temperature no doubt seeping into his skin and chilling him to the bone. Leonard knew he was usually slightly chilly just on the Enterprise, which was in a perpetual state of 70 degrees Fahrenheit. On the planet, whose average temperature was 55 degrees Fahrenheit, his Vulcan was likely freezing.
"Oh, my poor little angel," he cooed, shutting the door behind him with his hip and beginning to undress Spock, who complied with however he was moved with relaxed obedience. "Why didn't you tell daddy you were cold?"
"Work," Spock pouted, and Leonard chuckled, leaning up to press a kiss to the tip of his nose and making Spock giggle before folding the last piece of the Vulcan's clothing and setting it on the sink counter with the rest of them. He'd already gathered his pajamas and the thermal underwear he'd wear under them to keep him warm, as well as the knitted sweater he'd given him last year for Christmas. Spock wore that whenever possible, and now was no exception.
"Well, you're not working now, baby," Leonard told him, turning to the bath that he'd already drawn up and making sure it was the right temperature. Satisfied, he helped Spock into the steaming water, feeling him slump with relief as soon as his body came into contact with it. When he was settled in the bath and starting to play with the water, splashing and giggling all the while, Leonard began lathering his hair with shampoo, massaging his fingers into his scalp and the tense knots of muscle in his neck and shoulders. It took little time after that for them to finish their bath, and soon enough Leonard was powdering a squirming Vulcan and securing his diaper before blowing a raspberry on his exposed belly button and making him squeal. After that, he had to wrestle him into his thermal underwear and then his onesie, but he was only all too happy to let Leonard pull the sweater on overtop of it. It was certainly big on him, the collar loose around his throat and baggy around his middle, the hem almost reaching his mid-thigh, and his sleeves nearly covered his fingertips, but he looked positively precious in it, especially with his beautiful brown eyes and stuck-up hair and flushed cheeks. It made Leonard smile.
"C'mon, pumpkin, I've got a present for you," he said, guiding Spock out of the steamy bathroom and back into the main part of their rented quarters, stopping briefly to let Spock snatch up his stuffed sehlat Commander Cookie and to give him his pacifier before showing him the paper and the crayons and watching his face just light up. "You like it?"
Spock nodded happily, and began bouncing on the balls of his feet with excitement while Leonard got him a few pieces of paper and the pack of crayons, setting them on the ground and watching with amusement as the Vulcan immediately lowered himself to get on level with the strange contraptions. They both knew Spock knew what paper and crayons were, but like this, they were just a new thing to explore, and Leonard always enjoyed watching him discover new things.
A few minutes later, Spock had pages worth of colored scribbles, ranging from nothing to something resembling an actual scene, and Leonard knew it would be a good night.