This might have started out with an intention to keep Christmas "normal" for the children but they are old enough by the sounds of it to have a proper conversation with you about your relationship having ended and that as an adult you are entitled to choose to spend your time with people you want to spend your time with.
Tell Dad that he is welcome to take them all to a restaurant for a Christmas meal as he doesn't have the space at his house, you can enjoy a special wine and a Christmas film then you can have cheese and coldcuts in the evening with just your kids at home.
Add to that: have your own Christmas dinner with the children on Christmas Eve night so you don't have to spend Christmas Day cooking and washing up but you still have the cold cuts for the Christmas Day night. Or do something completely different!
We have my stepchild (who's now an adult) on Christmas Eve. Every year. That's just how our own traditions have ended up. So we used to have family over for a gathering at ours at tea time on Christmas Eve after a day of opening presents, playing games etc. We'd never do a roast as who wants a roast 2 days running?! It would be seasonal and festive and as she got older, my step daughter chose the menu and helped to make it all - Christmas sushi year was a highlight!
Another choice for next year is to plan a holiday away for Christmas - we've got friends who do this each year to avoid their own family dramas (not an ex).
Or invite over other family or friends (especially as the children get older, could be their partners, we had one of my uni friends who was a foreign student come for a few years so she wasn't alone over Christmas, elderly neighbours and relatives, church friends of my parents, anyone and everyone BUT NEVER ANYONE'S EX!