I didn't know kids could join Mensa. I'm not going to pay the membership fees for any of us to be tested and possibly join, although at least when I was a kid I qualified (who knows about now!)
But you have access to their site even if you haven't joined. This is the site that I was particularly pleased with:
It has games, links to other cool sites, a feature of the month (on Pi this month), and a true understanding of what gifted kids are like. For example, you can click on a game link and play a word-unscrambling game, and then also color the picture on the page. That's cool. Often "advanced" stuff forgets that kids are still kids.
So this is definitely a go look. The adult mensa site also has games you can try. I am going to pass or I'll get addicted, but there are lots of word games. Very cool.