If you get WiiWare games (the ones you download off of the internet through the Wii) they are all pretty inexpensive (around $10).They also have it so you can get past Nintendo games (Mario I, II, III, Original Donkey Kong...)
Mario games are always kid friendly. I have an 11-year-old, and that's what we got him.
Mario Party 8 is great for the whole family. It has mini games, and most of the games aren't too hard to play. Also, Wii Play and Wii Music are great for kids too, plus, they're also fun for adults to play as well. The down side is that they're a little on the expensive side. Mario Kart Wii is awesome. It's the #1 sold game for the Wii, and you'll have hours of fun with it. You can play it as a family, play the computer, or even play people all over the world.
There's also a Nerf shooting game. It's fun, and it costs around $35-$40. I hope this helps.
If you have a game store in your mall, you can normally get used games there pretty cheap, and they're guaranteed to work. Also, the games that are $50 in the stores you can get brand new on Ebay for about half that price. That's one of the best routes to go.
If you see M&M Cart, it's not worth you time or money. My children, my husband, and I all hated that one.