My impression is that there is not too much free Christmas music. (Please correct me if I am wrong.) In Germany, during Christmas there are often performances of Engelbert
Humperdinck's opera "Haensel and Gretel". It contains a wonderful duet by Haensel and Gretel, when they are alone in the forest at night frightened by strange noises of wild animals and the wind. They sing a prayer. I tried to make it into a trombone trio, with trombones 1 and 2 being the kids and the bass trombone the "base". Of course it shouldn't be played like a heavy trombone section in a 4000 seat symphony hall but more like singing my 4 year old son into sleep (which by my own experience is quite possible with the trombone).
The trio can be found here http://faculty.econ.ucdavis.edu/faculty/schipper/humperdinck.pdf
. It can be freely used under Creative Commons - Attribution.
If you like to hear the original, here is an example https://youtu.be/91h8uxAUOKI?t=25s