I can second most the the suggestions so far.
You could most probably look at some of the games marked 8+ and 10+ if the theme appeals to the child/children in question. In my personal experience, especially once they can read, children can pick up on game rules and concepts fairly easily if they are interested in the game. Just looking at the CCGs aimed at children, such as Pokemon and Yu-Gi-Oh!, these are fairly complex games as each card changes the rules in some way.
My suggestion would be to go to your FLGS and see what games interest them and check Boardgamegeek.com.
GameBrowser - Sorted By Rank | BoardGameGeek should be a good place to start.
You can also lookup the games they enjoy and look at the You might Enjoy tab.
Phaezen
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