The only difference between Adult and Children editions (it's not that uncommon for fantasy books, really) is the cover. The price usually doesn't change, but can.
It allows adults to place the books on their book shelves without having to fear that their friends find out too quickly that they're reading children books. Very mature, indeed.
In the case of Harry Potter, though, the Adult covers were not as über ugly as the alternative. That goes for the seventh, too.
By the way, I once saw a very beautiful cover for Harry Potter, but I can't remember which one it was. I think Greek edition. If anyone knows what I mean and has it, pics please.