Tell them sensible adults prioritise the best interests of children in any complex emotional situation, and therefore you're doing just that.
The weird thing about messy families is that people will squawk when you first lay down boundaries, and then in no time they're the new normal and just accepted. So if you set it out now, firmly and as it's going to be - no apologies, no suggestions, just THIS IS THE DEAL, then after an initial wailing and gnashing of teeth, everyone will fall into line.
Your kids deserve a happy family Christmas. That's what matters. Your respective parents and new partners can visit you when suits all sides.
The problem is that you've not stuck to your guns long enough to normalise things. They're in the habit of emotional blackmail succeeding, and to them it feels like they're expressing their needs. But their needs have ruled your lives, from the sounds of it, all through. Why should they rule your own children's lives?
Christmas traditions are great. Your children should be allowed to start some. In their own home, with their own tree and presents and meal, without stress and travel and adult drama.
Your parents are being massively, massively selfish. Shift the expectations - two years of doing that consistently, and it will be the new normal. Promise.