I was also not given a heads up; I was like a deer in the headlights when my eldest DS said, "We met Dad's friend...[OW]?" I was cleaning something at the time, and I had to turn my back on him so he wouldn't see the grimace on my face.
I agree with the others. It's hard to believe, but over time, you will care less and less when her name comes out of your kids' mouths. Honestly, when my kids talk about CommandOwife now, it's typically to complain about her, and I don't foster that environment. I expect my kids to behave respectfully and politely, so I typically deflect their remarks or say something like, "That must have been difficult to deal with." So far, she hasn't done anything so terrible that I have needed to get involved; my kids just don't care for her, and I remember feeling worried that they'd like her more, would think she was wonderful, etc. They liked her at first, but now, I think they just tolerate her. Both she and my ex are slowly but surely showing my kids who they are.
Just keep a smile plastered to your face, say "That's nice!" and then change the subject: "So, do you want to go to the playground later?" Kids really can't tell when you're steering them away from a subject deliberately, so that's your best bet that allows your kids to talk to you without creating drama for them or excessive pain for you.
You'll get through this! (((Numb)))