If you have a Gamecube, Super Smash Bros. is a pretty good choice. In my experiance, kids love that one because it's button-mashy enough for them to play it, but still fun. They also like the characters in it (Pokemon and Mario characters especially). Some of the new brawlers like Ninja Turles and Batman: Rise of Sin Tsu (I'm assuming on the Batman one - I've never played it) are also fun for kids because of their simplicity. They are a bit repetitive though, so if they have short attention spans, they will get bored pretty quickly. Beware of some overly complicated "kids" games though, like Mickey Mouse's Magic Mirror Land or whatever it's called. I made the mistake of buying that one for my neice because she is a huge Mickey fan only to find that it is way too complex for her. The best I can do with that game is set it up in kids mode for her, which makes Mickey run around and do things on his own, but that loses it's charm pretty quickly.
-Matt
