First Christmas separated from H, what is fair?

[Sothisisitis]

Two children.
Ex Husband is off on Christmas Day and Boxing Day. One child’s birthday is on Boxing Day.

What would be a fair arrangement?

Ex has stayed in the family home which the kids are attached to. We co-parent.

I agree with this. I get the impression from the OP that the parents are thinking in terms of what is fair for them, the adults. But its really about what is fair for the kids. And what is fair for the kids is what they would prefer, especially if this is the first Christmas after their parents split.

Hopefully the parents know their kids well enough to have a solid idea of what their kids would prefer. And to know if their kids have the security to know they could express a preference to their parents and that this would be respected and honoured without question quarrel or grudge.

I think this sounds a reasonable plan.
I know it sounds back and forth but if you live close that really isn’t an issue (as long as the kids are happy) and like a PP says they may want to see both parents on Christmas Day, mine certainly do.
Your child having a Boxing Day birthday does really complicate it.
You might have to accept that your family don’t get to see him on his birthday. Do they always see the other child on their Birthday? Grandparents that live close wouldn’t necessarily see my DC on their actual birthday but we include them in a birthday tea close by.

I always would take children’s views into account but I would never pressure them to make a decision as it can be really difficult for them as they don’t want to upset either parent.

Also be careful as what we decided years ago is still what happens now pretty much.