IMO...
If your child is that easily swayed as to want to be a "witch" after watching a movie as obviously based in fantasy (or reading a book of the same type), I'd have to say that they've been well over-sheltered.
As was said earlier, these books have no basis in any religion.
These stories are metaphores as is true with almost all literary works. The back stories are also of high importance... It includes anti-discrimination messages, self esteem through hardship messages and teaches kids that everyone can be a hero. Of course these messages are available in other places....
How are most forms of casting much different from prayer? I know start another thread.... The point is that loving and acceptance are taught in these movies and that IMO makes them worth my children seeing.