At what point do you say "ok then, you don't have to go today"?
Kids stay with their dad 2 nights a week.
Eldest (8) is always happy about going and while he has some moans about unfair things at dad's I suspect he does the same about here.
Youngest (5) is another matter, he rarely wants to go, will say throughout the day that he doesn't want to go, will say mid-way between visits that he doesn't want to go. But, I understand (from ex and ds1) that he is fine when he is there - although given how much he doesn't want to go I do wonder how fine he actually is.
If he was meant to be going anywhere else then there is no way that I would be making him do so, week in week out, sometimes having to physically prise him off me.
I think he deserves to have a relationship with his father, I don't think he is old enough to make the decision not to go, and so I make sure that he does.
But at what point is it his choice?
(hopefully it will never come to that point, and he will start going happily and this will be a pointless question!)