Mario and Sonic at the Olympics is good for all ages if you don't already have it, although a Wiimote and Nunchuk for everyone would be a good idea with that one - makes it a lot more fun.
Mario Party 8 is another good one as the girly girl can be Princess and stompy boy can be Mario or Bowser, it's like Snakes and Ladders, but with mini games thrown in and some devious tricks to play on your opponents.
Carnival Party Games and Guiness Book of Records both look quite fun, although I haven't played either of them yet.