Things only have to alter if they had been spending a significant amount of time together and doing lots of things together.
This is just one morning, Christmas morning! And you think that should change because a new partner comes along, take a step back and try to look at it logically and you will see you're being hysterical for no reason.
He's literally just going to share their Christmas morning with them opening presents, it shouldn't be a big deal for you in any way at all.
My ex nearly always came to mine for Christmas morning, I never invited his girlfriend at the time (even though we got on great), It genuinely didn't occur to me, and my ex never asked if she could come either. I think it would have ruined the vibe a bit for the kids if she was there because it wasn't a family kind of relationship with her.
She would normally come over later in the day for a little bit when my ex came back to collect the kids for a bit and we'd exchange gifts 😂
But later on in their relationship and after they split up my kids really started to notice and piece together the times when my ex put her before them, and they judged both him and her for it.
So if you're in this for the long haul like you think you're going to be, do you really, REALLY want his kids to remember when their dad stopped coming over for Christmas morning, they will almost certainly think it was because either he picked you over them or because you asked him to!