There are some positive here, that don't happen in other co parenting situations
He has sorted out the childcare on his watch, by himself and not defaulted to you
You may have been busy, had plans etc and its his watch, he has sorted his babysitting and taken that responsibility on
So may co parents don't sort it out themselves and resort to the other parents, then the other parent has plans, cancels their plans etc
so I can see that side of it
Sometimes for a night going to spend it with grandparents is just going to happen, maybe your dc don't really want to be there, but it not going to hurt them. What it might do is make them appreciate when they are with the people they like being with
As long as you are around as emergency parent contact, that maybe would have been wise from your ex