T.~
I know the problem from the other side of the coin, but I feel your pain through my husband. His wife left him before we met and we've been doing the holiday routine for over 10 years now. Two of the kids are grown and gone now, but the remaining child is disabled and we will get to spend holidays and summers with her forever. :)
Our children are 2 and 5 and we have a regular Christmas for them on Christmas day. Our schedule with my step-daughter is that one year, she's with us on Christmas day, and the next is after Christmas. Each visit is for about a week. She gets her gifts from us when she gets here and they get to be happy for her.
It's not an easy situation, but it's a good learning tool. Be sure to stress the reason for the season and the gift of being happy for another's joy. That may not be a magic pill, but with time and repetition, it's a good lesson.
Good luck.
~J.