No, this was scheduled by the therapist during my time. My ex is alleging that neither he nor I let her know so she is not liable to share the costs. But yes you are correct, she used to schedule activities on top once she knew he has the session on a particular day and time.
That's my point: you said when you informed Mom about the scheduled appts, scheduled during your court ordered parenting time, she would find reasons for the kids not to come. People have been known to lose custody over pulling crap like this.
You have to be meticulous in your record keeping, and not stoop to her level. I am not implying that you be a doormat. I am stating that you should be the best person you can be - don't be petty, be honest and be direct, not angry.
It may take a while. You may not get what you should. But it gives you a better chance of having a healthy relationship with the kids, long term.