His children are not your responsibility particularly if they show you no respect and their parents aren't grateful for you looking after them
Why should mum be grateful? As far as she is concerned, they are spending time with their dad. If he isn’t available, it is still his responsibility to make alternative arrangements on his time. The ex doesn’t need to be grateful he does that. It’s what he has signed up to.
What SBTLove said - you will not be providing any childcare because ‘you’re not their parent’
Parents don’t provide childcare. They are parenting when their children are with them. If you allow yourself to see it as ‘babysitting’ then huge resentment of mum develops. She is allowed to sit on her backside and do sod all if that suits her. Or go out and get blind drunk and sleep with half the local football team. She is not needing to get home for the babysitter - her children are with their other parent.
If DH can’t because he is working then their Mum can look after her OWN bloody kids!
No, their father can do what he has already agreed and if his new wife has found a pair of balls, he can arrange childcare/holiday clubs. You don’t go back on an agreed period of contact with your children because your wife won’t help out. You man up and deal with it, remembering you are a parent and that is what parents do. If he doesn’t like it, he renegotiates with his ex for future holidays or less time overall. He doesn’t change an established arrangement because it no longer suits him.
she will NEVER have them extra and is always trying to palm them off more while maintaining she is super mum
Except in cases of emergency, why should she have them longer than has been agreed? Are all separated mums ‘palming off’ their children when they are with their fathers? Presumably your OH is super dad yet he is the one palming off his children on to you!