Today while searching for an easy piano version of one of my favorite Christmas songs - "Still, Still, Still," I discovered this site - www.easybyte.org that contains numerous arrangements of Christmas songs and other music in the public domain arranged by William Wallace. The arrangements are in pdf format and many include electric keyboard midi samples.
A few other sites with elementary level Christmas music that I have found include:
laytonmusic.com - simplified primary songs
In my studio, I print these off, put them in sheet protectors in a folder arranged according to level of difficulty and allow my students to check them out from my music library during the holidays. Its been fun to see how the boost in confidence for students who check out the same songs the following year and can easily sightread the pieces that seemed like such a challenge for them the year before.
If you know of any other piano Christmas music sites please leave a comment and I'll add them to the list.