You're a better mom than me. When my kids first mentioned that they met the slunt, I had to grit my teeth and turn around. I think I managed to squeak out a "that's nice" before I completely diverted the conversation on to another topic. I'm better with it now, but those first few times were like nails on the biggest chalkboard ever invented.
My thoughts are that you don't want to be so enthusiastic about OW that your DD thinks you're all honky dory with this new situation. At the same time, you can't grimace or act pissed off when she talks about her either - the line of communication has to stay open so that she will let you know when OW shows her true colors - and trust me, she will.
My kids have virtually no opinion about the slunt. She's the price of admission to see their father. On occasion, they will say that she's mean or that she screams at her kids too much or that she is not the person they want their father to be with. They have NEVER come home gushing about her and telling me how great she is and the sweet things she does for them.
I think your DD is going to have the same experience. If she didn't come home gushing about OW and didn't come home with all sorts of doo dads in her hair and her finger/toe nails painted up to the nines, this OW is not interested in making a bang up impression. That's how it was with my kids and the slunt. I was prepared for a whole show and all sorts of phony bullshit, mostly heaped on my DD because she was the girl. This bitch couldn't even put on the show. I think she's so insecure and self-centered, that the minute my kids expressed any sort of dissatisfaction with their father's choice to leave and/or they said one word about how much they love their mother, she stepped back and took any and all focus away from them. I think she is not a big fan of my exFIL because, the one time she met him, he looked at my DD and said, "you're so beautiful - you look just like your mother". That one went over like a fart in church. But, it's the classic OW mentality.
My friend, it sounds as though you may be looking at the same sort of scenario. Unless she's really stupid, she won't be mean to your DD when your ex is around, but she also won't be overly anxious to win her over.
Bottom line - most OW don't want the insta-family. They liked it better when the fantasy bubble was still in tact and the WH was able to focus all of his attention on her during their secret getaways. She loved the fact that the kids, the mess, the nasty bathroom habits, the clothes on the floor, and the dishes in the sink were left at home with the unknowing wife. Well, congratulations OW - you got the whole kit and caboodle now. Enjoy!