OP, I can well imagine this must be very upsetting for you and for the children, however this is 100% on your Ex.
He is dumping his children on his new partner.
He needs to return them to you and you increase your CM claim.
He clearly is with this woman for free childcare and she didn't sign up for it, or she has had enough and is putting down boundaries.
Many men do this, but not all.
She has her own child that she would like to take out on her own.
She is not wrong to want to do that.
He expects free childcare.
Your children have two parents and it is 100% the responsibility of you both to provide it.
Not to expect his new partner with her own child to want to take care of 3 children on a day out.
This is on your husband and you would be wise to speak to him and not to blame her or even mention her.
You and he have no business commenting on how she spends time with her child at the weekend.
I know it reads as harsh but this is the reality when you split.
He's just another man who got hooked up and had a baby with the first woman he saw, as he needed childcare after you split up.
She no longer wants to play ball so both of you need to step up and look after your children between you, not expect his new partner to give you both free weekends to work and socialise without your children.
This is unpalatable for many who expect women to suck it up, but its the truth.
Your children, your responsibility.
I really feel for your children.
Its a miserable childhood when you get caught in the crossfire.