I'm normally the first to get irritated when people come on saying they should get christmas over their childless colleagues but actually in this situation I would have worked the shift for you OP. Its a fairly unique and difficult situation and you have both been fucked over by management who apparently don't care about whether colleagues have good working relationships or not.
However for some of the posters on this thread who have been vile about childless women:
I don't deserve Christmas less because I was born unable to have children (or if I had chosen not to have them)
I am not a less worth member of society because I am not producing a future tax payer and possible care worker
I already contribute towards the future of society through my taxes.
There will always be another parent thinking their christmas day is more important than mine. Always. It not just this christmas it every single christmas in a relentless drone of how much less worthy we are. The colleague probably hasn't agreed because other colleagues demand every year that she gives up christmas not just this year.
And to those who think they are somehow more deserving members of society because they can successfully have unprotected sex:
There are a lot of bad parents out there. Society would actually be better if people thought about whether they should have children before they had sex. Having a child regardless does not make you a more valuable member of society than someone who recognises parenting might not be their strength and makes a mature decision not to do it.