Disclaimer: I don't own Frozen or Sherlock.
John Watson ran alongside his reindeer Mary, following the adults along the frozen surface of the ice. They reached a spot that looked both good quality and still harvestable for his smaller size and strength. He was still a little kid, but one day he would be big and strong and work long and hard along the other men. Pulling up a little block of ice and getting a bit splashed in the process, he loaded it onto Mary's little sled and sped off toward Arendelle, where two other young boys lay quietly in their beds…
…Or at least one of them was. Mycroft Holmes was fast asleep in his room of the castle and burrowed under the blankets so that the only thing peeking out was his snow white hair. The room was still and quiet until a mop of black curls popped up over the edge of the bed. The younger of the two princes was not known for sleeping through the night, and his elder brother was about to get a rude reminder of this fact. With a mischievous smirk, Sherlock launched himself up onto the (slightly snoring) lump in the blankets.
"Mycroft!" he ordered, "Get up!"
After giving an incredibly manly and not at all girlish squeak of indignation, Mycroft pulled a pillow over his head with a groan. "Sherlock, I know there is no chance of you going back to bed, but could you please, please, at least let other people get some sleep? I have important things to do tomorrow." There were only three years between them, but at eight years old, Mycroft liked to sound older than he was. He was going to be King of Arendelle one day, and future Kings surely always had important things to do, right?
"The moon is up and much too bright, so I am up and much too bored. So, Mycroft, you have to move your massive rear end out of bed and play- I mean, entertain me." Sherlock, who also fancied himself older than his five years, did not play. He experimented and was entertained. In fact, one of the only games he did play was deductions, which he would only play with Mycroft.
Mycroft groaned again and shoved his little brother off the bed, telling him to entertain himself. Thinking for a moment, Sherlock's face lit up and he climbed back onto his brother.
"But My, I want to play deductions…and maybe experiment with snow?" Sherlock knew he had Mycroft when he finally sat up in his bed. The eldest Holmes never could resist a chance to practice (show off) his ice powers.
Soon enough, both boys had abandoned any pretenses of maturity and were playing like the children they were. Mycroft laughed as Sherlock raced around the ice, tumbling into a pile of snow. Together, they rolled a snowman, which looked more like a snowgirl, and Mycroft gave it a voice, saying "My name is Molly, and I love to play and helping with experiments." Sherlock giggled and started an experiment with Molly the snowgirl.
The next thing they knew, Sherlock was hopping around, being caught by snowdrifts from his brother and calling out deductions he'd learned about the staff. Desperately trying to keep up, Mycroft slipped on the ice. Not noticing, Sherlock kept jumping and laughing.
"…And then I noticed that she had mud on her shoes, which meant she'd actually been with the gardener, can you imagine? Catch me My!" He leapt just as Mycroft was sitting up.
"Sherlock!" He yelled in a panic, desperately firing a blast in the direction of his baby brother to try and catch him. Unfortunately, his aim was off and Sherlock went silent as he was struck in the head and tumbled down a drift.
Mycroft ran to him and cradled him in his lap, calling his name again. Nothing changed but a streak of white in Sherlock's messy curls. He lay silent and still.
"Mummy! Papa!"
AN: So here we are. I decided I really wanted to see this crossover written out, so….here it is. Just so we're clear:
Sherlock: Anna. The younger sibling and the more energetic and troublemaking of the two. Fairly obvious choice.
Mycroft: The Iceman. Again, fairly obvious.
John: Kristoff. Sherlock's new friend and the one to at least try to knock some common sense into him.
Mary: Sven. This is not meant as an insult, nor am I comparing Mary to a large working animal or calling her an unimportant character. Sven is very supportive and caring toward Kristoff, and for a long time they were all the other had. They are very close and each other's best friend, and Sven ultimately has a role in supporting both Kristoff (John) and Anna (Sherlock) and pushing them together throughout the movie. This is why I chose Mary for this role, rather than say….Lestrade, or Mike or something.
Molly: Olaf lives for helping, and can be very self-sacrificing. He sticks around for Anna, even when things get dangerous, because she needs him. Does this sound like a certain pathologist we all know and love?