The thing about children's books, good children's books, is that they manage this trick of never talking down to the reader. I got the impression that the creepiness and sense of horror I got when reading about the Other Mother's hand, the monstrous transformation of Other Father, etc. are not things kids focus on when reading, and indeed, Gaiman himself has commented on it. The kids I've know have all seen it in a more adventurous light, as Coraline's character is a trickster-hero, a puzzle solver, and in identifying with her, the reader is too.