I would say that most of what they have is pretty good quality, and they do have a pretty wide variety of items. Some things might seem a little expensive, but HP items seem to be about the same price even when purchased elsewhere. I have seen lower quality (non-HP) souvenirs sold at other theme parks for much higher prices.
What I wanted was a really neat "evil" shirt, like one that said "Death Eaters" or "Dementors" or "Voldemort" or something, but without pictures of the actors or movie scenes on them. And I found a Death Eater shirt that I really liked. T-shirts are reasonably priced, under $30.
Had the wands been made of actual wood, I would have definitely bought one. But they probably would be more expensive too, it would probably take more work to carve those designs into wood than to cast the resin ones, even if a machine did all the work. Not to mention they would probably not be made from the actual species of trees described in the books/movies, sourcing some of those in adequate sizes and quanitities might be difficult. The wands they do have are nice though, they are good reproductions, and they come in very nice boxes that make them even more worth the price. Be careful when waving though, they are not unbreakable.
I only remember seeing a couple of witch's hats, and they were pretty expensive but looked very cool. Save your money if you plan to buy one.
My personal favorite souvenirs to bring back were the photos I took of WWoHP, they are easy to share with everyone and I like to play with editing tools to make it look like an even more awesome place than it already is. Honeydukes/Zonko's has a nice selection of smaller treats that would be fun to bring back and share as well. I think you can also send postcards from the Owl Post and postmark them Hogsmeade, your child's friends might enjoy getting those in the mail.
