I just bought Just Dance 2 for myself and a few 'kid' games for my 4 year old daughter. She has no interest in the games that are for her and last night we played Just Dance for an hour and a half until she got a cramp in her belly. She absolutely loves it! I'm getting the first one now and I think there is a 3 also. I wouldn't recommend Dance Dance Revolution for the little ones because it's hard to hit the arrows they don't really pay attentino to that but copying the dance moves is so easy and fun. My 4 year old rocks!
One thing to remember though. During the "Monster Mash" track it shows Frankenstein and he does a few things like taking his arm off..this may scare your two year old. Otherwise it is a really fun game.
I play it with my 6 year old daughter. I did have to take out some songs for her due to lyrics and some motions they had her doing that were too mature for her. I can't remember the name but one was about getting tipsy and drunk. It was just my personal preference that she doesn't need to hear. The choice is yours on that call.
I did buy the just dance for kids and we have done a couple of the songs. It has real people doing the motions on it and she seemed to enjoy doing the dance and it was easier for her.