I have 2 DC but they have different dads. Both dads are involved.
I’ve had them every Xmas eve so far and they’ve opened presents with me on Xmas morning, then gone their separate ways with their dads. (Or sometimes the youngest stays with me if his dad is working).
Oldest DC, DD is 7. Her dad has asked to have her Xmas eve this year and bring her back to me after dinner.
If I didn’t have the youngest I would agree, he’s never experienced it before and he’s a good dad. I’d find it a bit sad and difficult but I can deal with it. The problem is if he has DD, then I have a young DS opening presents alone with me on Xmas morning without their sibling. Then I’m swapping them as one leaves and the other comes back 
I grew up very close to my sibling and not sure if it’s clouding my judgement, but this doesn’t sound nice for them.
AIBU to say no, it’s not fair on the youngest DC to take the eldest away? Youngest is autistic and old enough to understand they would normally be together.
Sometimes this “swapping” ends up happening for contact in the week as hers is set at weekends and his changes. DD doesn’t seem too fussed but DS always gets very upset.