Potter-world encyclopedia is one of the best.
Parents need to know that this is best described as a Harry Potter encyclopedia and online forum. While advertising is far from the focus, Google ads appear on the main page, some "Lexicon" related retail is available (t-shirts, bags, mugs, etc.), and links to Amazon for J.K. Rowling's books are placed throughout the site. There are three levels of access to this site (guest, read-only, participant) and the discussions are definitely monitored by staff as well as self-regulated by contributors.