I wouldn't tell anyone I thought might need therapy about my T :-) About my experiences with my T maybe but not who.

I don't understand why your T accepted your son as a client. That would be a conflict of interest to me as your son might be talking about you and "influence" what your T thinks about you plus you have talked about your troubles raising your son, etc. I don't think most T's are going to accept individual clients from the same family?

I'd just explain to your T you aren't comfortable with him treating your son/the girl anymore and want him to terminate that relationship as it feels too much like a conflict of interest to you (and is because of the bgirl from heck). I wouldn't lose faith in my T until I knew exactly what was going on and he knew how I now felt about the situation. Like you said, you weren't thrilled with sharing him with your son even, well, tell your T it isn't working for you! He's either going to try to wrangle something so he can keep all clients (in which case I would terminate and not be one of them) or he's going to respect the whole thing isn't working for you, his primary client and pass them off to a friend/other therapist.