Most kids are drawn to other kids' voices, but not all - my niece, for example, prefers hearing grown-up voices. My kids love to hear kids sing, so here are the two CD's we're currently getting the most mileage out of recently (and singing around the house most often):
The Broadway Kids Sing Broadway - more info: Here.
This one is in the car and the boys look forward to rides in the car so they can sing and laugh with this one. Being broadway songs, there is an occasional word or two that some might want to avoid for little ears, but overall, we love this one. They also have a "Best of Broadway" one that's pretty good too.
Songs That Jesus Said - Keith and Kristyn Getty - more info: Here.
What are some of your favorites?