Well, I'm not sure that there are that many good educational games, but there are some that I think a 5 year old could figure out.
It really depends how good they are at video games. If they're still on a basic level, it might be best to stick to 2D puzzle and platformer games. The older NES and SNES Mario games on the VC (Super Mario Bros, Super Mario World) might be ideal for this, as well as any version of Tetris or Dr. Mario. Kirby's Epic Yarn looks like it might be a good choice, also.
If they like racing games, you could try Mario Kart Wii or a similar racing game.
If you think they can handle a 3D game, you might want to invest in Super Mario Galaxy, and/or some of the Lego adventure games.