He's not a child, its just his vampire body didn't develop properly and still behaves in a new/child vampire stage.
hes young for a vampire ig but they said hes grandpa age in human years
He's not a child, in human years anyway. In vampire yrs he's a bit past the that child state. It says 150 yrs is needed for a new vampire to grow and needed the guidance of an older one. However, during that time, his auntue who was supposed to be with him was in a deep sleep, leaving him alone. Thus, he wasn't able to properly mature. 50 or so yrs later here he is, grown but not yet mature enough as a vampire. Thanks to our ML's super power blood and ehem sperm, he was able to develop his body AND his power as a vampire.
I'm sorry (´･_･`)? What? He's a child???