So to summarise, the way I explain it is to bluntly tell him that it’s not his children’s needs that get to dominate everything. My children have needs to and due to the age gap, he should both respect and understand that. It is not remotely personal to any of the children - but based on the fact that teenagers vs young primary school children have very different wants and needs. As parents we respectively have a duty to fulfil our respective children’s.
And that’s where the misalignment happens. Your children neither need or want to see his, so in you keeping them separate (as I am doing), we are fulfilling their needs.
His children (and my partners children) on the other hand want to see your / my children/ us and probably ask a fair bit. So in their eyes, their children’s needs are not being met.
So our children get our needs met. Theirs dont.
I don’t know, I think this happens in all families. My nieces and nephews used to spend lots of time in my home/ with my DC when they were younger. Once they hit about 16, it stopped. Initially my children were upset/ asked for them. And all I would explain is that they were older, and growing up and that they were loved, but it’s what happened when children got a bit older. They wanted more independence, not go to their aunts house to play with their little cousins. Accompanied by lots of jokes about smelly/ grumpy teenagers. My kids were absolutely fine! Now some of my nieces, nephews are early twenties and visit occasionally, but that’s about it. I predict that once mine are young adults, relationships may once again pick up with them.
It’s part and parcel of growing up, it’s good for children to understand they are not the center of everyone’s world! And that others have wants and needs also, that they need to respect. Gives them a healthy dose of reality and humility.