Saw the basic idea for this on tumblr. (someone had posted about how they wanted a christmas fanfic about how the Team finds out that Marvel is really just a kid). I thought the idea was fun and interesting so here's my take on it.
A Young Justice Carole. [A/N: Not related to A Christmas Carole whatsoever]
...Captain Marvel was a good guy. He was happy, supportive, agreeable, sociable, flexible, loyal, and more. But the team could not help but notice his... peculiarities. Sure they would say things like, "well all super heroes have their issues," "No one's perfect," "Everyone's a little different," "At least he's not as messed up as Batman," (to which Robin would respond "HEY! ...yeah...") but at the back of their minds, they couldn't shake the thought that Captain Marvel was just too... innocent.
No one speculated very far on this thought. No one voiced it either. And no one dared to think it over the telepathic link (and if it ever slipped out, it was firmly ignored). All the same, each team member had their own theory or two.
Kid Flash thought he aged backwards, like a whole Benjamin Button sort of deal only caused by an experiment gone awry.
M'gann thought he was being sensitive to Robin's age (the young little bird).
Conner thought he was stupid.
Artemis thought he had lived his entire life in some sort of constructed reality or cursed manor where he was forced to live his childhood over and over again, and so he never grew up mentally.
Kaldur thought it was better to not think about as it was personal, and it, probably, was just him trying to relate to the team.
Robin thought it had to do with a medical condition (amnesia, split personality, something like that), or possibly some traumatic event caused him to revert to a childlike mental state.
They were all wrong, as they found out, one fateful day
…
T'was the week before Christmas. And all through the cave. Not even our Kid Flash had a snacking crave. The team hung out at the training room where - they hoped that good old Batman soon would be there.
A mission to protect the living and dead was what the team had envisioned lay ahead. M'gann, with her cookies, Robin with his cape, the team waited for word of evil's escape.
When out from the zeta tube, arouse such a clatter, all the team members looked to see what was the matter. Away to the tube, hurried our Kid Flash to Captain Marvel in his red suit and gold sash.
The man with a smile and innocent glow, confused the team members, for they didn't know, that this man, his muscles, and deep brawny speech, turned this way magically whenever he did beseech. [SHAZAM]
Captain Marvel grinned, excited as a child. The team ignored him, scared of his mood turning wild. But his joy would not fade with holidays near. He giggled and exclaimed (and confirmed the teams fears).
"Hey Kid Flash! Hey Robin! Hey Kaldur, Artemis! Oh, guess what you guys? Oh, Hi Connor, Martian, Miss. You'll get presents for sure! You're so nice after all. I've been good all year, so when Santa comes I'll make such a haul!"
Then the puzzle pieces began to align. They stared as though the man was out of his mind. "Is this guy seven? He believes in Santa Clause!" The telepathically shared thought gave them pause.
"Could it be? No way!" They all realized at once. Marvel was a kid, not a lune or a dunce. They looked at each other, stunned and ashamed; they treated him like a servant or no-name.
He was always generous and well meaning. Now a way to apologize the team was scheming. Meanwhile, he stood there not knowing their thought. His only concern the joy holidays brought.
"Captain Marvel?" Rob asked, smiling with charm, "What is your name?" his question meant no harm. Marvel considered and finally said "Billy," to him, their question was overdue and silly
"Billy, you want to have a party with us?" Meg grinned "For the holidays, for Christmas." Billy's admiration grew three sizes that day. T'was a dream of his to, with Young Justice, play.
"Yes! Yes! Yes!" He shouted with utter delight, thinking of no better way to spend this night. Together, they hung up holiday decor: tinsel, wreaths, trees, fake snow, ornaments, and more.
Billy spoke not a word, but merrily worked. The team felt glad he didn't take them as jerks. All of them now covered in festive red and green, The Batman suddenly walked in on scene.
His eyebrows rose in shock, bat-emotion? What? He even smirked as back, out the cave he strut, "Leave them to their party, no bad guys to fight. Happy Holidays, team," thought Bats the Dark Knight.
That was fun. I think I'll write cute stuff about the team in general again. :D and Daddy Bats cuz Daddy/Mommy Bats is the best!
PS. CAPTAIN MARVEL I LOVE YOU!