I went through this. When I split from my eXH my DC said they were glad the split was equal and that it wasn’t a one week on one week off scenario. But they were increasingly unwilling to go to their dad’s, and going with him on holiday was a nightmare.
It didn’t help that my ex just couldn’t deal with any of it, he would put pressure on DC and e.g. tell them how many times they’d called him when on holiday with me vs how many times they wanted to call me and how unfair it was etc etc.
I never forced them to go, but I always had conversations around them going. Then ex met a new partner and DC disliked her from the outset. She is also very adamant that eXH not have anything to do with any of his past. She never goes with him to see his own family etc, refuses to meet up with any of his friends, so from that perspective they lead separate lives, except she demands he spend as much time with her as possible. They had a child together and that made things worse for DC.
Things came to a head when DC was around 13, and eXH and his partner had a serious argument on holiday where she screamed at him in a public place, slagged me off, accused me of all manner of things etc. After that DC refused to ever stay there again.
Thing is that eXH has the ability to be a decent parent, but he seemed to lose sight of that. And he’s very manipulative anyway which just got DC’s back up. They’re now 18 and the relationship with their dad has improved. But they see him only away from his house, and have no relationship with either the partner, or the child she and eXH have.
At this age I do think it’s important to encourage the children to have a relationship with their other parent, assuming the relationship isn’t abusive. Because at that age a child just doesn’t comprehend the longer term impact of breaking down a relationship. But as time goes on and they mature you can also alter your approach, until they’re old enough to definitively make that decision for themselves.