Why not plan some christmasy craft to do with them, to decorate for the evening when you have people over. Make table decorations, place mats etc, so they are actualley part of preparing the festivities for your evening with family. They could be involved in preparing and setting the table etc.

Maybe also give them a special dish to prepare for dinner- that way you are having a special day together, but it is also helping to get ready for your evening.

I also think a special brekky or lunch, chilled out infront of a christmasy movie would be good too.