My kids and 13 and 14, and I would ABSOLUTELY not allow them to have anything to do with OW. Right now, they do not even know about OW. BD was only two months ago, and my H has not moved out (but he wants to really, really bad).
His relationship with OW is not out in the open, so I did not see a need to tell the kids about her. I have been hoping that somehow, the OW relationship would end and the kids would not need to know. I do realize that if he moves out to be with her or they start being more open with their relationship that the kids will have to be told.
I am prepared to tell my H that as long as he is married to me, I will not allow OW to have ANYTHING to do with the kids. I believe their relationship is wrong, and I do not want my kids exposed to it. Now, in the event that H files for D and is still involved when it is final, I would talk to H and ask that he allow the kids to get to know her on their terms (if they wanted to).
I hate this situation for my kids more that anything. When I was a teenager, my dad went through MLC and cheated on my mom. My mom was an emotional wreak, and she told me and my sister every sordid detail. I was the oldest, and Mom really depended on me more than she should have. Thankfully, Dad did not try to push OW on us. We never met her, all we knew was her name and everything that Mom told us about her. Dad and his OW did break up, but he and my mom never reconciled. I hated my dad for several years, but now we have a good relationship.