The following books are good:
For Piano Adventures: Primer pupils Piano Adventures Primer Level Christmas Book, and then for each level there is a Christmas Book. Don't go for a higher level as it's useful to learn these quickly! Each Hal Leonard Level also has a Christmas Piano Solos book in its Hal Leonard Student Piano Library series. There are also lots of other books and some attractive jazzy grade 1-5 arrangements by Pam Wedgwood, Heather Hammond and Martha Mier so do browse www.musicroom.com or go to Allegro or Daces shops in town. I look forward to hearing everything Christmassy!