I really buy more for what the family can enjoy together. Having said that:
We Bought These and Have no Regrets:
The Lego games mentioned above are great! My five year old loves them so much that we bought all of them. Two players for all levels so it is great to play together.
Boom Blox - This can be somewhat addictive to adults and can play up to four players. There have been a few times where the adults have played this one after the kids have gone to bed. Steven Speilberg was the creator and the graphics are very kid friendly. It really utilizes the Wii remote.
Mario Cart - You use the remote as a steering wheel and can actually play tournaments online. My five year old really liked that one a lot and you can play up to four players.
Mario Galaxy - As mentioned above, a really fun game for kids. Not a two player game, so it isn't as enjoyable for the father/kid experience, but the graphics are good and it will keep you involved while the other player plays.
Rayman Raving Rabbids games are really fun. Although it isn't a two player game, it is as much fun to watch play as it is to play, so it keeps the non-player(s) involved. They make you do some very crazy stuff on these games, but they are pretty kid friendly things. Good "shooter" games (Actually, you shoot rabbids with plungers or carrot juice, no bullets). Excellent game to max the Wii experience because it uses the remote in every way imaginable. Very silly stuff and I enjoyed playing it with my son.
Super Mario Party 8 - This game really depends on your/your kid's tastes, because it is somewhat like a board game for the Wii. It plays up to four players, which means my wife and I can play with our son (our daughter isn't old enough to play yet). Good stuff if you like playing actual games with your kid.
DeBlob: This one is very very kid friendly. You paint the town with this blob character after these evil creatures took all the color away. Stages are timed. Pretty entertaining game, but it is one player so it can get a touch boring for the non-player. Very good use of the Wii remote.
Rent, don't buy:
Shrek the third - Good kid game, but I was not increadibly entertained.
Ratatouille - Pretty fun game for the kids, but some levels were a little hard for my son. Pretty entertaining for adults, but I would suggest renting as opposed to buying.
Nicktoons Attack of the Toybots - My son is a big Nick/Spongebob fan, so this game was a pretty fun rent. Some levels were a little hard for him.
As an aside: It seems that most movie based games are between okay and bad, so I would always suggest renting before you even think about buying.
Playground: Good for the kids, boring for the "non-participant". This is one that my kid can play on his own, but it isn't very much fun to watch him play. You basically run around and do all sorts of playground activities (Paper airplane flying, RC racing, dodgeball, etc.). Good rent, but it is only one player. I wouldn't buy it.
A few games to stay away from:
Wall-E - WAAAAY to hard for a kid my son's age. Starts out pretty easy, but gets harder with each level. My son loved the movie, so he was really bummed about how hard it was.
Igor - Boring. My kid got tired of it really quick
Meet the Robinsons - Boring. My kid put it down quickly.
Paper Mario - Just not something either my kid or I could get into. Somewhat of a good concept, but it just didn't really deliver.
Mario Super Sluggers - Very good game for an older kid, excellent use of the remote. But, this one was a little over my five year old's head because of all the different aspects you have to control (different types of pitches, different swings, etc (and he is ridiculously good at the Wii for his age). It left him a little disengaged. Probably a better game when they get a few years older.
Balloon Pop: Somewhat like a tetris game, a little hard for my five year old and he was disengaged after about ten minutes. I found it pretty boring myself.