well, to be fair, a huge chunk of adult fantasy features children as protags, at least a t the beginning. This is because, however, most books in fantasy are about coming of age in some way, learning to cope with power, learning responsibility, finding there's a larger world out there, discovering and grasping your destiny - ideas identified with growing up. So it's a natrual response to have characters that start out immature, even physically as well as emotionally, and grow into the ideal. See. there's this thing called dynamic characters too. Which I think we should do away with. No one should ever learn from their experiences.
(quiz: guess where I turn sarcastic in the above paragraph)
Anyway, considering the ideas being used, why is it a problem to use children? Not all books with dynamic characters use children protags, but it's hardly a problem to do so since that's the metaphyscial idea being used anyway. Dynamic characters who change for the better, who improve themselves, are maturing. It's natural to signal it with a physical maturation as well. Of course, it becomes ironic if they become more emotionally immature, and makes quite a different statement with a static character. I don't think that readers or writers will ever tire of talking about growing up, even if it's just a side subject, because it's universal. You're always getting older, so we need to confront it. including it in your work doesn't make the work less interesting or good. In fact, ignoring that it happens would do damage to most novels.