Are you asking for children's literature, or YA?

The younger the better, I suppose. Although library and bookseller classifications have to draw the line somewhere, there's really a continuum of target audience ages. Anything that is widely read by children should count, regardless of how it's classified (although how it's classified may give a reasonable estimate of whether children read it, in the absence of real data).

Eliezer has referred to HP as ‘for children’ when explaining some of the changes that MoR (which is not for children) has to the background universe. But HP is often classified as YA. I would not want to be picky.

humanizing a villain that would predate on someone that young would not be wise.

That's an interesting argument. I definitely believe that children must learn that villains are humans too by the time they are old enough to commit acts of revenge that can cause significant harm. So certainly tweens (who will soon be old enough to join gangs, plan for future careers, etc) should read about humanized villains, while still reading about heroes who resist them. But very young children may need to classify people strictly into good and evil to successfully avoid harmful people. That's an uncomfortable idea to me, but I don't know enough about child psychology to rationally evaluate it.