The "O Antiphons" are special prayers (that can be chanted) for the days preceding Christmas (December 17-23). You can read about them here.
Dr. Mahrt is posting the chant along with beautiful recordings at the Church Music Association of America Blog.
Project for children: O Antiphon House
O Antiphon Coloring Page from Illuminated Ink
UPDATE: Jennifer Miller has a very detailed set of resources and ideas on O Night Divine.