I found this great website that has a bunch of great piano music for kids.  Most of the items I saw were free, and you could download them as PDFs, or hit "play" to hear them played.

It has pieces for different skill levels from very early beginner to some fairly complicated pieces.  The only drawback I saw to the site is that it is jam-packed full of ads, but I found it well worth navigating the ads, for the great quality content.