My 5-yr old son Xavier and I spent a while playing Assault Heroes, the latest game on Xbox Live Arcade.  It's a neat little top-down vehicle shooter, with tons of interesting little mechanics.  Fun game, though a little simple.  It clearly pushed all the "coolest thing in the world" buttons on my son though, with giant robot spiders and tanks and lots of stuff blowing up.  The fact that it is co-op is fantastic... there really is proving to be a weatlh of co-op games coming out (even if I have to wait a few more years before Gears of War or Resistance are really appropriate to play with my kids).

So many good games, so little time...