Well, this isn't a trading card game... Just like my last recommendation...
Boss Monster is a pretty awesome game that I recently discovered. Each player sets up a "dungeon" in an attempt to attract and destroy the "heroes" that come to town each day. It's pretty quick to learn (though the first couple of rounds you'll have no strategy), and it's very fun(ny). I suspect that even the 7-year-old will be able to learn it.