We aren't having yams, but if we were and there were marshmallows on the top, I would be tempted to decorate them with these cute designs from The Decorated Cookie:
I know that realistically that wouldn't work, but it is a fun idea! What will work is her plan to keep the kids busy by giving them some marshmallows and food coloring markers. You could even set them up with a copy of the step by step instructions she gives on her blog HERE.

