thumb prints made into Christmas lights. Easy craft to do with the kids. To enhance this, I might extend the strand each year, so by the time they are older, you have a long string of thumb print lights.
Art Projects for Kids: Snowflake Art Trading Cards. Punched snowflakes glued to painted watercolor paper. Love these - even little kids could do a series of these in diff winter/Christmas themes, and could gift them to mum and dad as gift tags:)