My stbexh has a habit of arranging activities with his girlfriend's kids during my (court ordered) contact time. The children live with me and see their dad EOW, half the holidays and once a week after school so he has plenty of opportunities for activities during the time they are with them.
Every time he arranges something it's always something the children really want to do and have usually asked to do but it's for a time they are not with him and we are often away so they can't go. This causes the children to rage at me saying how unfair I am, how mean, dreadful mother, the works. If I arranged stuff in his time I'd be told to fuck off in no uncertain terms (long history of abuse) and there is no chance he'd ever change contact dates to accommodate something I'd arranged for the children no matter how much they wanted to do it, even if it was a one off opportunity happening only on that date.
I feel like I'm always the bad guy and if we aren't doing something I need to arrange something so I have a valid reason to say no.
Contact between us is limited to emergencies only as he is abusive in communication so he sends these requests via the children. I refuse to engage with them being used as messengers though.
For context he rarely does activities with them during the time they are with him as he is usually working, sleeping or too busy with his gf's kids.
How do I deal with this?