My ex and I will be accomadating with special hlidays, if we know that a family member might not make it to the next one. Or because our sons has been in the hospital needing intesnsive therapy.
But he has
never said "change of plans, I'm going to the ballgame instead."
The other parnets, are obligated to taking the kids when they said they woudl. I'm assuming it is part of your divorce agreement who gets children when. Use that to fell bak on. And quite bluntly say."You said you were going to take the kids on these days:_______so, I made plans for these days_______, there are nonrefundable tickets." And leave it there.
Our exs get too comfortaable with us giving in on this kind of stuff, because thay know we will, eventually.
Just stick by your guns long enough to win this ddebate. Heck get DH to help run interference on the call!