We, the Cal Quidditch team, are available to be hired to host your Harry-Potter-loving child's quidditch birthday party.
If your son and his friends are into Harry Potter, our favorite 8yo birthday party was hiring the Cal Quidditch team to run a quidditch game for the kids. They provided all the brooms, hoops, balls, etc., and it was a blast. This was a few years ago, so I don't know if they still do it, but you can reach them via Facebook at https://www.facebook.com/calquidditch/. The Adventure Playground someone else suggested is also a great idea.