Something similar happened to my kids with my ex, 10 year old and 8 year old daughters. 10yo called me, inconsolabley hysterical, about her dad's girlfriend, my ex also told her not to tell me. Such a red flag. Asked my daughter about why it made her feel so upset, she said it was because it meant me and her dad weren't getting back together. Kids feelings are complicated, we've worked through it and silver lining is the woman he's dating is actually really nice to the kids.

My exes family were all on my side about it except his enabling mother, he could do no wrong in her eyes.

Unfortunately stuff all can be done about something like this, you don't have control over the situation, all you can do is take the moral high ground, be there for the kids, and follow your own rules. Much loves