If you've got a court order that lays down what is meant to be happening on Christmas Day, is there any way to say that if she tries to stop Madi from coming to your house she's in breach of the court order?
She sounds like a nightmare - I'd probably start saying about the children's best interests and that he's concerned about her welfare, with her manipulating, making life difficult for a 5 year old, bringing a friend to the contact centre when the whole reason she was there was to go with you and then making her feel guilty for going etc etc.
I really do feel for you - my ex tries to use the kids to manipulate; he's got a lot more contact with them at the moment than I'm comfortable with (he lives 40 minutes drive away and has even managed to get them on regular schoolnights) because he kept making a huge fuss about how many sleeps until she sees Daddy, was constantly taking them out to expensive day trips (despite apparently not being able to afford maintenance) which I couldn't ever afford to match, buying both kids expensive presents, taking her to MacDonalds, letting her have her own way about everything (almost no rules) etc etc...all the usual stuff to make a 4 year old want to be one place more than another. I've had to make some concessions, but have said in mediation that I'm not happy about him getting Heather worked up, that I expect her to have a stable life and to be fed healthy food, bathed regularly and put in clean clothes, that I think he's more concerned about what he wants and isn't thinking of their wellbeing and if he can't provide a stable, predictable, healthy life for them I'm not happy that he has their best interests at heart. Oh, and that trying to manipulate the situation using a four year old's emotions borders on abuse and I'll definitely not be agreeing to him having them more if he continues. He still has them a lot more than I'd like (and is still trying for more) BUT he's at least stopped upsetting my daughter to try to get his own way.
I really think your OH needs to put his foot down and tell this horrible woman that if she won't stick to what's in the court order, and if she continues deliberately upsetting this poor little girl and trying to make it hard for her to see her Dad, he'll be going back to a solicitor, as she's clearly not putting her best interests first. Ok, it might be a bluff, but it sounds like this woman needs the sh*ts put up her.
Is there any way you and your OH could qualify for legal aid? I'd never be able to afford this if I didn't.
Try to stay positive hon, even though it sounds like an awful situation, I can't believe there are women like this in the world.
xxxxx