How about.........

getting those really decorative muffin liners (snowmen, trees, santa, snowflakes, etc.) and baking the cupcakes in them. If they want them really Christmas-like though, maybe try buying a WHITE cake mix, making the batter like usual, put it in 2 separate bowl, and food colore to one batch red and one batch green. If you didn't want to do that, you could just add food color to the frosting and add holiday sprinkles. If you are really pressed for time, just make them as you would normally and get some of those "sugar decorative pieces" that can be found in the craft section at Wal-Mart. They have all sorts of really cute Christmas ones. Hope some of this can help.