Essentially, after tonight, I think this is where I am at:

I will not promote a R between my kids and their father's gf.

I have reiterated to them over and over that their father loves them very much, and that our divorce had nothing whatsoever to do with them.

My S15 knows the reasons that I don't like, trust or respect the gf - he gets it.

I told D11 that the gf had betrayed my friendship and that I am wary of her, but that I also trust my D to come to me with any concerns, or to go to another safe adult like her grandparents, other family, or her IC. I apologized to her if I had added to any stress she might have had. I want her to be happy, and comfortable when she visits her dad.
She did say that she did want to be able to have playdates with her bff (who happens to be the gf's D). I told her that she didn't have to worry about my feelings with that, I was glad she said something, and I would be OK for playdates at either home. I had always been open to the girl coming here, but I guess she wasn't allowed because I wouldn't let D11 over there without her father also present. That is the point that she looked the most relieved.