To be fair, I started off with a lot of sympathy for OP as I can imagine it's not the easiest to imagine your DP playing happy families with their ex. Because that's at the heart of it, isn't it? The DP and his ex are enjoying a happy family scene every Christmas morning while the OP feels left out and pushed to the side.
I understand those feelings, and I can well imagine lots of women would feel the same way. However, where I think the difference lies is that OP feels that she's actually in the right, and that changes need to be made to accommodate her hurt feelings. And that in a nutshell is why she's being called selfish. Because she is.
It's fine to feel a bit uneasy, but the adult thing to do is put the children first. They're only children for such a short space of time and it's just a couple of hours one day per year.
Expecting the children's Christmas to be changed to accommodate the whims of the dad's partner is totally unreasonable.
The OP has the right to ask her DP to change his children's Christmas to suit her. He's entitled to refuse. OP then has to decide whether it's a deal breaker or not - and that's her choice.
OP has said she can't see the benefit of her DP and his ex both being there for their children to open their presents. She's either being obtuse or is incredibly self-absorbed. Not many families manage this but if they do, it's wonderful for the children. And in this case the youngest is only 10 years old!
As a full disclaimer, my parents were divorced and I was really close to my stepmum. Probably closer to her than my actual DM. But it was still really nice that my DM and DF got on well together and got together on my birthday etc, it made me feel less tugged in different directions.
I think it would be entirely reasonable to talk to the DP to see how he envisions the future and Christmasses as their families blend. But to demand that he drops Christmas traditions with his children to keep her happy is just bloody awful.