I think you're following HPoirot's sitch, right? It sounds a lot like what his WAW is doing to him, regularly changing arrangements. Everyone has told him to set boundaries. Go to his thread for scripts from the vets.
You might have seen that my take is to be firm and inflexible. He can't take the kids at the agreed time? Well, neither can you. It's his problem during that period. When my WAW left the house, she said she'd need two weeks to get settled in the new apartment. I said "No: you take the kids after one week. They need you. You can arrange their bed in a week." We have almost no re-arrangement of schedule.
I believe that we teach people how to treat us. We set boundaries, we accept certain things, we make them regret treating us a certain way, etc. We don't even realize it. Now, he's testing this new S arrangement: can he change the schedule at will? The answer to him is yes. How will he react next time he has an opportunity that overlaps with his time with the kids? Will he think that he can always change things? Apparently so.
Probably irrelevant anecdote: Yesterday, I was texting a friend and he said his W had put their kids (S5 and D3) to bed, but they were constantly coming out asking for stuff, which the W couldn't refuse them. Well, no wonder they kept on coming out. They don't come out with him because he returns them empty handed.
BTW, is your H successful at work?
M39 D6 D3 (at S) S 2014-09 D 2016-09
"You can't start a fire sitting around, crying over a broken heart" - Bruce Springsteen.