sanemi was turned into a baby (5 years old) after he got hit by a demons attack, and Kagaya (the demon slayer corps leader) Disided to leave sanemi in your hands, you where not alowed to leave or go on any missions, so you had to be with sanemi in your room watching him play with his toys you gave him, but you didnt pay attention to him at all, all you did is polish your katana or read on your bed