His ex called this morning and she expects us to just do Christmas for DSC as they will be upset otherwise. They were talking about it over Christmas and upset they wouldn't get a proper Christmas with their dad this year.
Well it was her choice for him to have them over NY instead, whereas the original arrangement was for them to split Christmas Day and Boxing Day. She sounds like a loon.
Ex says we can't combine this with staying up for New Year's Eve as Santa comes at midnight. She always does party food Christmas Eve and Christmas dinner Christmas Day, leftovers Boxing Day, so that has to be the same here for them. She says they don't like the kind of food my family have for New Year.
Why on earth is he even discussing the plans for what goes on in HIS household during HIS contact time, in so much detail with her?
He needs to get a backbone and start to deflect all these questions and tell her to butt out. His children will get to spend time with him, Christmas Day will be replicated somewhat, they will get gifts from Daddy and what happens the following day with regard to food and tradition etc, is none of her damned business. 
If his children don't like your food the fair enough. Don't rise to it and see it as some sort of battle to be won. She'd love that That's what she's hoping for. That her children will feel alienated by being faced with food they don't like or recognise will feel to her like she's scored a goal against you. Just get them a pizza or whatever they like, bung in the oven and you do your thing. Offer them both. They can eat your food, not eat it, their choice.
AIBU to book us last minute flights and leave DP and DSC to it?
Actually, if everything you've said is completely true then no, I don't think YABU at all. You've been more than accommodating enough and his ex is really taking the piss now. If your DP won't stand up to her then you at least you can. Go and see your family.