Some conservative Christians feel that reading about witchcraft and wizardry can lead children to try to practice it and thus lead them away from God. In my opinion, this is silly. The Harry Potter books have nothing to do with organized religion and everything to do with morality. Magic isn't a religion for the witches and wizards in these books. Any child older than about three should recognize that these books are pretend, just as Jack in the Beanstalk is pretend.
Kids love to pretend.
These books help them do that in a big way. Kids are drawn into a slightly different world, a world a kid can imagine. It's very well developed and satisfies a child's need for consistency. The books also favor doing good stuff over doing bad stuff. All the villains either get their come-uppance or are going to in the end.