STBX just called me at work to give me crap (politely though) for discussing our situation and her relationship with OM with kids. I have tried not to do so, but sometimes the conversation regarding our futures etc does go that way.
I would keep it simple, BTM. "Believe me, W, I'm not going out of my way to make this any more difficult on anyone that it already is, but I decided that I'm no longer willing to lie to the kids to cover up your affair. I'm not going to intiate anything, but when they ask me a direct question, they need to know that at least ONE of their parents isn't going to lie to them anymore."
These are consequences that are hers to own, not yours to defend.