I am a mum of late teens and early 20's whose dad was an awkward sod, and also I was very regimented with their routines as it suited ME
I NEVER stopped any access for the various difficulties he put in my way.
I NEVER badmouthed him and never let anyone else badmouth him in front of the kids
I NEVER moaned that I did all the grunt work, washing, homework etc. I saw that as a privilege
I NEVER questioned what he had said to them or if he had said anything negative about me - I'm sure he did
I tried to introduce a new partner into their lives a few years after dad and I split up but it wasn't ever going to work for various reasons.
I am not perfect by any means and I've had my moments - but my kids are well rounded people who have all thanked me independently for NOT influencing their ideas and ideals about their dad
I was bitter, yes, that they saw their dad EVERY weekend and that we never had the opportunity to have movie night on a fri/sat night, or snuggles in bed on a sat/sun morning
We had mediation. We were encouraged to have a communication book. I wrote in all my important dates, family birthdays, grandparents, cousins etc. He had to do the same. If it was my weekend and it was his mums birthday, we swapped(where we possibly could). It was best for the kids.
Imagine what it's like for your poor kids if he's late and he's saying ' oh god, she's gonna go mad, we're late again' They will dread going cos they wont know how to deal with the fallout. You wont be doing any favours to your children if you let them take that attitude into their adult lives.
It really pissed me off that he was the Disney dad and I was behind the scenes doing all the continuity work.
But they now have a decent relationship with him. they don't see him that often, but when they do they enjoy it I think
Your ex (not your childrens EX - he's their dad for ever remember!) sees his kids from Friday night to Sunday night every other week- 48 hours out of a maximum of 336!
Cut him a bit of slack. Meet him halfway with this and be thankful that he wants to be in their lives. He sees another man taking his place as father and doesn't want to be forgotten