What if Ur was a frost dragon? Imagine that the magic she taught Gray and Leon was really a form of ice dragon slayer magic. In that case, we can assume Ur would probably have known Igneel, Metallicana, and Grandine. So there's a good chance that the young dragon slayers could have met too.
Two young boys trotted up the hillside, breathing heavily as they tried to keep up with their master. Said woman paused and laughed at her disciples. She stood at the top of a ridge, her black hair outlined against the sky.
"Gray! Leon! Hurry up! We're almost there!"
"Why do we have to go see Master's friend anyway?" the boy named Leon asked, wiping sweaty silver hair off his forehead. His companion shrugged.
"How should I know?" Gray asked. White momentarily obscured his raven hair as he slipped his sweater off. "It's too warm out. Who does Ur know that lives here?"
Eventually the trio came to a halt in front of a cave. Ur knelt down to better talk to her tiny students. "Now, I want you two to be good for Igneel. He'll be looking after you for a couple of days while I go on a mission with my sister."
"Why can't we go with you?" Leon whined. "We could help!"
Ur laughed and ruffled his hair. "No, it's best you stay here. Grandine and I are going demon hunting; it's too dangerous for little boys." Then the tall woman stood up and yelled at the cave.
"Oi, Igneel! The boys and I are here!"
The answering voice was like nothing Gray and Leon had ever heard. It was a roar, a cross between the rumbling of an earthquake and the crackling of a fire. Soon afterwards there was a series of thunderous footsteps from the cave. The boys hid behind Ur in fear.
First to appear were the mighty red paws, large enough to hold a human like a toy. They were followed by a body the size of a small house, with a tail that stretched back into darkness. A crown of horns sprouted from the dragon's skull, something fluffy and pink rising between them. Golden eyes stared worriedly from under massive brows that were creased into a worried frown.
"It's a dragon," Gray whispered. Leon nodded mutely.
"Pleased to meet you boys," the dragon muttered distractedly. "Ur, just give me a moment will you? I appear to have misplaced my little salamander."
The pink fluff on the dragon's head let out a stifled giggle. Ur smirked. "Well, I guess I'll have to ask Metallicana to look after them then. I can't leave Gray and Leon here if little boys keep going missing."
"I'm not missing!" The pink fluff jumped up from the dragon's head, resolving itself into a boy a little younger than Gray and Leon. "I'm right here!" The boy slid down the dragon's side and landed in a tumble.
"Natsu," the dragon sighed, "What did I tell you about today?"
Black eyes stared sadly at the dirt. "That you had friends coming over, so you want me to behave. Sorry Igneel."
"Good." Igneel nuzzled Natsu's head gently. "Now, this is Ur and her sons Gay and Leon. Ur has business with Grandine out east, so the boys will be staying with us for a couple days. You be good with them, you hear?"
Natsu nodded. Leaping forward, he grabbed Gray with one hand and Leon with the other. "Come on! There's a pool in the back of the cave where we can go swimming!" The pink-haired boy dragged his new companions quickly inside.
Igneel smiled and turned to Ur. "Are you sure it's wise," he asked, "Raising those two without telling them what you are?"
Ur spread her wings with a grin. "I want them to be good with other humans. Your boy's gonna have trouble that way someday Iggy."
The fire dragon sighed. "I wish you wouldn't call me that."
"But I like to. Someone's gotta keep the title King of the Fire Dragons from going to your head." Blue scales glittered like crystals as Ur unfolded into her true shape. The frost dragon nuzzled her warm, red friend.
"Now, be good to my boys. I won't have you scaring them like you did my husband."
"It was your fault for taking a human mate," Igneel muttered. Ur didn't hear him; she was already in flight.
The boys, meanwhile, were having a grand time. Gray and Leon had never been to a natural hot spring before. True, they found the heat a little too much, but they were having enough fun that it soon ceased to bother them. Natsu had challenged the two young ice mages to a full on water battle. The three were splashing and laughing so much that they didn't notice Igneel come inside. At least, not until a dragon-sized tidal wave swept across the spring.
"Wheeeeee!" Natsu shouted joyously, riding the warm surf. Gray and Leon weren't as quick to adjust and ended up being swept away. This resulted in the pair lying on the side of the pool, coughing up warm water as Natsu laughed at them from between Igneel's horns.
Competitive as always, Leon cooled his hands with ice before splashing back with all his might. He was joined a second later by Gray. The pair produced a large wave of super-cooled water that swept over Igneel's head, pulling Natsu down. Golden eyes widened in surprise, then narrowed. The dragon grinned and turned to his son.
Over the next two days the three boys became fast friends. Igneel smiled as he watched them wrestling on the floor of the cave. All three would be good dragon slayers when they grew older. Leon's competitive nature would lead to great learning; Gray and Natsu were strong and pure of heart. Igneel was proud of the boys, even if only one of them was his own.
Natsu and Gray flopped apart, panting. Both little balls of energy were finally exhausted, their game over without a clear winner. Leon took the opportunity to challenge Gray; the silver-haired youngster didn't have as much stamina as the other two. But before Gray could respond, Natsu tackled Leon and pinned him to the floor.
"Nu-uh," the pink-haired boy said, "Gray'n me aren't finished. Nobody gets to beat Gray before I do."
Gray snorted. "What makes you think he could beat me Pinky? Or that you could either? I'm the strongest."
"No, I am!"
"No way, it's me!"
And thus the three boys resumed their playful scuffle. Igneel chuckled. Fire and ice… It didn't seem like they should go together, but they did. The boys would provide balance for each other, just as he and Ur did. Speaking of which…
"I'm back!" Ur marched into Igneel's cave, a massive grin on her face. A purple bruise spread across one of her cheeks, but other than that, she was unharmed. Leon and Gray immediately abandoned their tussle with Natsu to hug their mentor.
"Were you good boys for Igneel?" Ur wanted to know. Both children nodded.
"Can we come back again?" Gray wanted to know. "We like playing with Natsu. And I still have to get him to admit that I'm the strongest!"
Ur patted Gray on the head. "Of course we can come back. Now, why don't you go say goodbye to Natsu while I talk to Igneel?"
Gray and Leon scampered off. Sighing happily, Ur turned to her friend. "How'd you do it Iggy? Gray and Leon almost never smile. Especially Gray, not that it's a surprise after how I found him."
"It wasn't me," Igneel rumbled, "It was Natsu. My little salamander knows how to make anyone smile."
"Maybe Metallicana needs to send his little brat to stay here next."