I don't see why a 5 year old couldn't play some VC games from shop channel too... MarioKart 64 is nice and easy and I know some 5 year olds that can handle it fine. People - remember... 5 is the new 8, LOL!
Super Monkey Ball (especially the part games)
Stay away from Rayman - blech
I have a 5 year old girl and she of course is a pro on Wii sports in bowling and boxing. It's quite funny how well she can box. She will destroy any challengers human or CPU.
Anyway, Rayman was too hard for my 5 year old. But Super Monkey Ball was fun for her. Not necessarilly the main game, but a majority of the Party games were easy enough and there are a lot of them. (100 if I remember.)