I STRONGLY recommend that you find a Blockbuster or somewhere where you can try out the games.
We bought Mario party 8 on lots of recommendations, but my kids (8 and 10) hated it. They said the gameplay was better on the Gamecube. Ditto for Mario Strikers.
Carnival games has really poor graphics. You should not pay more than $20 for this game. My kids thought it was fun, but I thought it was very poorly designed.
Check half.com. I rarely buy brand-new games. Check game.faq for game reviews.
Games I/we enjoy:
Mario Galaxy - EXCELLENT! I have fun watching my DD play.
Mario and Sonic Olympic Games - fun, fun, fun!
Guitar Hero - it's is very cool.
Call of Duty - my 10-year old DS loves this game. But, there is shooting, some blood, death (I think).