This game has certainly stolen the show in our household. My 3, 5, and 8 year-olds love to play this game, and the gameplay in most cases is not too difficult for my 3-year old (however, he probably plays more games than the typical 3 year-old).
The game has a lot to offer, with just about every level in each world feeling different from any other. It's a good game if you can stomach the hefty price tag that new games come with these days.
The game does directly integrate with Nintendo's online universe, so your kids can be exposed to messages from other people throughout the world (not real-time chat or direct messages, but posts that people make in the form of pictures, text, etc), and the game encourages you (optionally) to post messages after each level. It would appear, however, that the messages are moderated, so I haven't seen anything other than friendly and fun posts from a diverse crowd.