since your kids aren't old enough to play Mature rated games...there are some that can be suitable for kids that are coming up for the hoilday season...
Need for Speed : Underground 2 (rated E for everyone) -racing game
X-Men Legends (rated T for Teen due to blood and violence)
Spider-Man 2 (rated Teen for violence)
Teenage Mutant Ninja Turtles 2 : Battle Nexus (rated Everyone for mild violence)
However, if you do have gamecube, I would actually recommend more games for the gamecube since it's actually a videogame system glued to younger audiences for the games of all ages...
super mario sunshine
mario kart : double dash
Billy Hatcher and the giant egg
Mario Power Tennis
and afew more to name...
Xbox and Playstation 2 are actually glued to older audiences for more mature games, as because mature games sell more because of violence, blood, gore, nudity, etc...
you know how popluar they are these days... lol