The difficult thing about the Harry Potter series is it gets darker and darker as the books go along.  When the books were coming out, there was enough time between books for kids to mature and be ready for the next book, but if you start your kids on the first book when they are just mature enough for it, they will want to read all of the books, but likely won't be mature enough for the later books.  I found it a little difficult deciding when was the right time to introduce them, but it has been done and they enjoyed them.