I am looking for a game that I can play with my kids ages 9 and 6. The rules of course need to be easy to understand and easy to remember.

Eventually I'd like to go deeper and play more sophisticated games as they get older, so I want to find a game that will pique their interest. It would be even better to find a game that is extremely simple to begin, but is extensible for later depth.

So my question is, can someone recommend a simple, suitable, fun game, or point me to a site that caters for minimalistic games aimed at younger kids. Thanks.