Stupidgrinonmyface I am sorry that your Dad has died and that you didn't get to spend Christmas with him after that first one.
However, that's not the situation with the Op, her ex picks them up after lunch on Christmas Day.
Also, there's nothing to say that this wont change as the children get older if they want to spend Christmas Eve and morning with their Dad and the afternoon with their Mum, but this situation is all about what HE wants, not what the children want or what is best for them. He doesn't care about that, he's just bullying the OP.
NO she doesn't want them to go to their Dad's Christmas Eve & morning, partly because she doesn't see why she should miss out on having that time with them (and I agree) as she does all the actual parenting of them while he has a guest appearance one night every fortnight this little contact is of his own choosing AND because the children don't 'want' to go to their Dad's normally, though they have a good time when they are there (2 nights a month
).
The OP says she accepts this will probably happen and she loves her kids and encourages them to have a good relationship with their Dad, so I am sure in future years this will happen IF it's what the children want.
This year, however, is all about a bully who just wants to pull the strings and frankly 'No, you can pick them up after lunch if you want to see the chidlren on Christmas Day' is the only right answer here, because if she caves into his threat of losing them entirely, then she's just going to be in for more of his bully boy tactics.