If a christian parent wants to let their child read Harry Potter, in light of some of the controversy, or because of the subject matter, I would hope they would read the books themselves before deciding one way or the other.
Just the same if a high school kid came home with the assignment to read Mein Kampf, a wise parent would judge the ability of the child to properly grasp all aspects of the book, and the discussion that was going on in the class room.
There are PLENTY of good books out there to get a child to read, and interested in reading. A parent should never have the opinion "that as long as they're reading, all is ok." They should be actively involved with the inputs into their childs life.
As the computer saying goes:
GIGO=Garbage in garbage out.
I’ve read all the books and they are pretty harmless. The source of magic is never made clear. They are not very deep reading although OOTP reminded me a lot of working for a large corporation.
However, I have seen no evidence that Harry Potter books encourage kids to read anything besides Harry Potter books.