Well there's the obvious whack of Mario games and re-releases on the platform.
All depends on your kids play level. My 6 year old step-son plays almost anything I do, and plays well.
I don't much like the Sonic games on GBA, but that's because I grew up with the originals...and the GBA titles are terrible in comparison. Your kids might like em though.
The Metroid games...again, depends on your kids level of play when it comes to video games.
There's Dora, Madagascar, Monsters Inc games etc on there too for the younger of them.
The GBA library is freaking huge...