Again, Limewire is the best, just make sure when you go to the Limewire site that you download the FREE version, and whenever it asks you to upgrade to a newer version, say later. There are tons of kids music on there! Good luck and have fun, music is so great for kids, and thank goodness we don't have to pay 24 dollars a cd anymore these days, especially when we're not even sure if we like all the songs on it.
Just type in kids in the title and you will come up with tons of songs!
PS. It's got great downloads for adults too if you want to find music for yourself- there's everything from Tchicovsky (my daughter loves the ballerina song) to Madonna!