I can somewhat understand that the yuki-onna girl's mind is that of a child, go figure what her actual age is.
But when it comes to the vampire, that sentence actually goes around in reverse. Go figure what her age is, but it surely isn't anything below 18. That makes her interaction with these children, and the snow-girl, to be pretty darn weird.
I think the vamp character will just crack this sort of manga apart in the long run. Adults should be adults, even if they are bored as all hell.