Monster doesn't necessarily have to mean supernatural. It could be a human killer or something similar.
Child can be anywhere from actual child to young adult.
Claymore, for a short time. Two childs, but /a/ doesn't like the second one *cough* Raki *cough*.
There is an anime/manga about a man with the head of a tiger, who can't remember anything. He protects a prince and princess twin from a fallen kingdom. The name is Guin Saga.
I liked it, till the point came where the princess fell in love with some worthless fucker, which they rescued , thus (in my opinion) betraying their savior.