Why is it selfish to want to spend time with your partner and your children? I reckon Christmas is a long enough celebration to do that and have time with other relatives too.
Pick the bits that matter to you and work around that. Some people see Christmas lunch as sacrosanct, but are willing to have people over for Christmas tea or go to in-laws for tea. Others want the whole of Christmas Day to their immediate family but like having the huge extended family sort of gathering on Boxing Day or New Year's Day or any day in-between. We don't go elsewhere on Christmas Day, but usually spend the whole of one day over the Christmas period with my in-laws. It's still as special - mother in law makes a special effort with the food and table settings etc, the house is still decorated, the children can take Christmas toys up to show their relatives and to play with, we still sit around after tea and play mass family card games. We also do a 'return' invite later on in the week, often on New Year's Day, when my husband's parents, grandparents and one or more siblings come to us for the day. They are brilliant get-togethers and still special and fun times despite the fact they are not Christmas day.
From what I can gather, the OP was not about to leave anyone alone on Christmas Day. Her in-laws are quite able to 'do' Christmas for themselves that year (I don't know if there are any siblings involved?). As long as she makes sure they see lots of the grandchildren over the Christmas period (assuming they live close enough), why does it actually have to be Christmas Day? Or if they live near enough, it could even be part of Christmas Day.
Having a family is about establishing your own traditions - that might keeping some you've grown up with, but it can also mean new traditions that are special to you. So that might mean new ways of 'working' the whole Christmas period and not doing something just because that's the way it's always been or it's expected of you. It might even mean a bit of compromise, 'we'd like Christmas Day this year just the 5 of us but would love to come to you/have you over next year'. In-laws have got to give and take too!