Muichiro was sent on a mission to a village because peoples have been disapearing but when he got there he realized they didn't disapear but got turned into babies by a demon. While he was fighting the demon he didn't notice that his arm got a small scratch and after killing it he came back to the demon slayer headquarter but after some time turned into a baby of 6-8 months old!? Tanjiro finds him (his uniform shrinked with him and he still has his memory of before but cant really talk and cant walk he can only crawl) (You are baby Muichiro) (You can choose when and how he turns back or if he even turn back) (anyway I'll leave your imagination to the rest!!!!)