Op it hardly matters what the 'norm' is, the fact is this is how he wants it to be at the moment and if everyone is still amicable then it sounds quite nice to give them one morning together.
Regardless of your status you have still only been together 3 years, not long really in the grand scheme of things, you don't live together, whether you want to and are waiting for the dust to settle with both your ex's, I don't know.
Does his wife know about you, do his children ?
I think what you are asking is about the permeance of your relationship with him, is he prepared to merge your lives together and proudly put you first. We don't know that, but him putting you above his children sounds self absorbed, whether some people never spend any time with their ex's is irrelavant to your situation.
Push him and you may find answers you don't want, concentrate on your relationship with him, moving in together maybe would be the next step, I mean you want the very best for your own children but you can't expect him to neglect his own children for the sake of your family unit and you.
Being a stepmom requires great diplomacy and I think you maybe need to expand a little on the empathy side of your nature.
And yes many women put children first and foremost above men, whether married or not, that is how it should be. Wives, husbands, girlfriends, boyfriends, all should realise parenting is more important, you bring a child into the world and they should be your priority.
I'll get slated for that by some, the me brigade always have something to say about that, frankly I've seen many children that are more grown up than the parents.
Yours was a selfish question, what did you want for him to stay with you Christmas morning with your children or were you thinking of shipping yours off to the ex and having a Christmas morn solely dedicated to you.
Maybe have the conversation about merging children and introductions if that has not happened yet, but just don't throw the dump the children and put me first on what is one of the most important days for children and parents.