In short, my kids (4 years old and 7 years old girls) enjoy it very much and it's just inexpensive and fun.
The USB dancing pad:
http://www.dealextreme.com/details.dx/sku.4234
The software:
http://www.stepmania.com/
Zero complain, and actually, I've something similar many years ago but it was a parallel port version and the quality was just far worse. This one is USB (no driver required) and it is much better designed. Highly recommended for your kids.