Ideas for Christmas Eve Family Church Service | eHow
From Dec. 1 to Dec. 24th each time you spot your child serving others you put coins or dollar bills in their pouch. On Dec. 24th you count the money and allow them to choose an item from the Compassion's gift catalog to give to someone in need.
The article is actually about making kids who don't want to come to the separate children's worship service want to attend it, but I think it could be easily adapted to make kids feel welcome in worship overall when you don't have a separate service.
What a great tradition to start.... Have family put money in mason jar throughout year. At Christmas time, choose someone to bless (anonymously). On Christmas eve, deliver by Ring and Run. Must read the book "The Christmas Jar" it explains how it all started. I love this story!