What would monster girl do if she find human orphan?
No, it's been confirmed that, amongst others, Mamono do not prey upon human children too young to have or even understand sex. They may even act as a mother figure towards the child, or wait until they're both old enough.
I wonder at what age would a boy become a fair target for a mamono…
Also, since mamono girls become active really young, and many pick their playmates as future husbands… would they try seduce a boy their own age while they are still children?
Most likely they would become playmates/friends of the boy they like. Though they will give litte hints of sexual acts here and there, but enough so that it won't be obvious. Then when both of them grow up to be adults, now is the time for them to strike. Look at the kappa for example.
But, at what age would the girl claim him as a husband? Low teens? Mid teens? Late teens? Twenties?
Well, since you asked....I'd say low teens, myself. Given that they (mamono) usually start at 10 years old themselves, this might be considered a display of restraint, depending on the family dynamics.