I have 5 between 18 months and 4 yrs. We did snowflakes. The kids just coloured plain white paper (scribbles) and I folded them and cut them toi create snowflakes.
I also took pictures of each kid beside my xmas tree, had the photos developed, and made a frame with construction paper, backed with cereal box cardboard for strength. The kids used xmas stamps i had with an ink pad and marker to decorate. (I only did this one with my 3 and 4 yr old). The parent loved it.
I also cut out green xmas trees, had the kids point to where they wanted to decorate with pom-pom ball, and i used hot glue gun to attach.
an idea i was thinking of doing was cutting out a large christmas tree from brown craft paper, and doing a large scale paint project (in diapers or something to avoid big clean up)
or getting the Sears toy catologue and helping the older ones create a wish list or letter to Santa with pictures of toys they like.