This is part of the issue. Characters can be complex and have a clear good/evil alignment. They are not mutually exclusive options. Yet many people seem to consider works with a clear good/evil dichotomy to be "simplistic" or "childish."
I think blank and white stories are simpler. When you only have two sides to things instead of the infinite shades of gray, it's by definition simpler. And you said it your self, characters can be complex and be clearly good or evil, and to me that's what will determine if a story is childish or not. The more believably complex the character's motivations are then, generally speaking, the less childish the story will come off. If the villian is "I will destroy the world because it's just the way I roll" and the hero saves the world for the same reason well that's obviously childish. If maybe a villain slaughters thousands in an effort to revive his dead wife and the hero want's to stop him, even though the wife is his sister, but he has his own family to worry about and will do anything to save them, that's a little less childish... not necessarily good motivations
Istar
Myth Weaver
Sage