Tuxpaint, just for fun. Tuxmath for...math. MinecraftPi for both fun and (by using the Python interface) learning to program. Scratch to learn to program, including having kids write their own games. Beyond that...look at the resources on this site by the RPF and read the MagPi.
My little sister is right now playing a super adorable puzzle/adventure game called Machinarium, I think it's from some independent game studio (maybe dutch? not sure), it's a really clever game actually, can definitely recommend