You are not being unreasonable to be unhappy about feeling second best, but if you look at this from his older children's point of view, maybe they already feel second best with him?
He's left them and their mum for another woman, with whom he has a child and another on the way. That new family have him the vast majority of the time - he's not with his older kids for the normal, day-to-day family life. How much worse would they feel if he couldn't spare them any time on Christmas Day?
I think you would be perfectly reasonable to insist on some ground rules - ie. if he says he will be back at a particular time, he is back then, not hours later. The split of time between you and your dc, and his older dc has to be equitable - it wouldn't be reasonable, imo, for him not to be there in the morning, when your dc is opening their stocking, or for when you all open presents, and for Christmas dinner - but you might have to look at how you schedule the day, so that he can do those things with your family, but also have a decent amount of time to spend with his other children.
As his older dc are teenagers, they are unlikely to be up at sparrowfart, to open stockings, so perhaps you could ask him to spend the morning with you, have Christmas dinner at lunchtime with you, and then go to visit his older dc in the afternoon for a few hours, then come back in time to do bedtime with your dd, and spend the evening with you.
It's also worth remembering that it is a good thing that he wants to spend time with his other children - that is the decent thing to do. And I appreciate it is not easy for you, but you do need to be the adult about this, and accept that, as you are with a man who has another family, he does have responsibilities to them too, and you can't expect him to ignore those, especially on Christmas day.