D.T.
There is really no reason to tell your children there won't be any Christmas. This year could be the Home Made Christmas where you make things for each other. You could start a tradition of re-gifting as well. Each child gives something they have out grown to the next child in line.
There are lots of places that can help with both the spiritual and commercial aspects of Christmas. When I was growing up my mother rarely had money for lavish gifts. She bought essentials and we accepted them graciously. Our Church played Santa as well. I could count on getting a robe and a Bible for Christmas and I was so happy to know they cared.
Register your children with Toys for Tots, The Salvation Army, and other groups that have Angel trees in the stores and shopping centers.
Really, the little kids don't need much and if the older kids want that x box or personal dvd player then they can earn the money for it by doing chores for the neighbors.
Whatever you do don't blame their father. I know you have a court order for child support and by all means call an agency to enforce that, but you can make due without having a ton of money for big ticket items.