I have two little ones, a 4.5 year old boy and 7.5 year old girl. This is one of the few games that they can actually participate in and not feel frustrated when playing. In fact, they actually have tons of fun and I think is their favorite Wii game as far as replayability (is that a word?). The mini games are short and for the most part easy to play but sometimes difficult to master which is okay for them. There is also a dude before each game that will show you how to hold and move the controller to play that particular game.
They also enjoy collecting all the prizes and being able to upgrade from the small to medium to large to extra large. Each game has a prize theme like robots, military items, stuffed animals, musical items, etc...
The tickets earned from playing the various mini-games can be used to play some of the showcase games like the Claw(to grab a stuffed animal), Pushing Coins(where you drop coins to potentially get more tickets), and more.
After playing Carnival, my kids and I feel like wanting to go to a real carnival until I realize that with a real carnival, I usually have to shell out some major buckaroos.
Minor issues: Like some of the other reviewers, I just wish the characters are more customizable like the Mii. Also, only a few games actually allow for true simultaneous multi-player like horse racing, pop the balloon, and duck hunt.