I picked up a copy of John Thompson's Christmas Carols in a charity shop the other day for 10p (clearly unopened) and it's one of the best I've come across for young beginners!
I also like Christmas Carol Time by Fanny Waterman - solos with optional duet accompaniments, which are really easy. We have good fun with these when I have two pre-Grade Ones back to back. I get one to come 10 minutes early and tell the early one to get picked up 10 minutes late.
Bastien is great for those of those between Grade 1 and about 4.
Myself - I have that big red book - "The Best Christmas Song Book Ever".
EDIT - I forgot about Laura Shur - "Christmas Tunes For Three" - six hands, published by Novello (special order) Some of these are on the Trinity list for six hands - I can't remember what level. If you enjoy utter chaos, these are brilliant to do with pupils - and a real hit at pupil concerts! I haven't done it this year, as it does take a fair bit of effort on the teacher's part, and I'm busy, but if you've time to spare for extra lessons it's very rewarding and a real laugh. Don't put a broader than average child in the middle, though...