compliment whatever you decide to put on them.
candles can create a centerpiece all by themselves. let the kids choose candles of various sizes and colors. let them mix the heights and colors of the candles however they choose. as an accent have them place colored marbles between and around the candles. the light will sparkle off the marbles when the candles are lit, creating a pretty glow.
greenery looks rather plain all by itself. to spruce it up, add a string of colored or white lights. a touch of sprayed on snow can be added to the tips as well to give it more of a winter feel.
how about adding some 