Straight from GoNintendo's Nintendo Rep

As for New. Super Mario Bros. the official word from Nintendo of America is that Koopa Kids are NOT in the US version of the game (I have no idea if they are in the Japanese version or not), but Bowser Jr. IS in the game.

As I said, this is straight from our rep, and it is very sad news indeed! While I don't mind Bowser Jr. , I was really looking forward to the return of the Koopa Kids. How could you not love Ludwig Von Koopa!