If you have Frys Electronics in your area try going into their store and find someone in the game area. They are very well versed in what games are available in the different formats, and they usually have the best prices and quality dance pads. My kids are 3 and 5 and love playing DDRMax, there is also a DDR Disney that is a little harder to find but a blast. For younger kids you can reduce the difficulty (how fast/complicated the steps are) and that tends to make it more fun. For the little one we turn off the scoring feature, she just loves to jump around on it.
Eye Toy is another fun way to play games, the kids love Groove, which is another dancing game. It is a fun way to get moving on a Saturday, and has been a huge hit at many birthday parties.