Character turns into a kid and is found by their team/family/crew/etc after the fact. A classic set-up for a kidfic, except there's two things that are different this time around:
They don't have their mind wiped. The kidifed character is fully aware of their past, their present, and just how small they are now.
When they find the team and desperately try to communicate the situation... No one believes them.
The team aren't doing it out of cruelty, they're doing it because, well... if a random child came up to you, claiming to be someone you know, just turned into a child, would you really believe them?
Maybe in this world it isn't possible. Maybe the team hasn't even encountered something like this before. Maybe they have, but the situation is just so absurd they can't believe it.
Maybe someone from the team believes the kid. Or starts to, after a while.
Maybe the kid is losing themself in the process. Maybe the mind-wipe is just delayed, and, eventually, they will become a normal kid, just as whatever caused it in the first place intended.
Maybe their memories are slipping away as they stamp their feet and grab their friends' pants in tiny fists and say over and over again that it's me. Can't you tell it's me?
Maybe the team pieces it together. They look at this tiny, angry, desperate little thing and finally see their friend.