I would ask your 12 year old what she knows already. When I asked my 10 year old daughter, she knew everything - even their plans to get married. Apparently she had even been speaking with the other man on the phone. When I found out this I went nuts and told her that mom was leaving me because she's in love with someone else. My wife was furious and her family rallied behind her but at least my 100% perfect record of never lying to my daughter is intact. She not only knew but was burdened with the secrets. My daughter is in my corner forever and she should never have been involved in this fling by my wife. I accepted her accusation but the bottom line is that I didn't destroy the family. My daughter could have a few more weeks of not being sure what's going on (before they get married) or her absolute confidence in my truthfulness could remain intact. I chose the best long term option because when my wife's relationship with this creep breaks up, I want to be able to show that I never betrayed my daughter.

I agree with Steven and the counselor....she is at an age where she deserves to know the truth, you don't want to lose that trust between the two of you, or you could have problems with her as a teenager.

I also think it would be a good idea to tell the truth. Yes, she is at a tender age, but whe will appreciate you treating her like an adult and not a chold. Just explain you were trying to protect her and didn't want to say anything bad about dad but you have thought about it and it is best she know the truth.
She will appreciate you being honest with her, even if she doesn't show it in the beginning. It's not always easy being a good parent, but some times we have to take the lumps as well as the pats on the back.
Good luck!

Why should we help hide their lies. I like what Steven said about asking her what she knows and take it from there. Who's to say the kids will turn on him. If they do it's because of what He did so that makes it his problem. Kids seem to love parents no matter what they do.

I explained it to both of my children. More to the 19 years old than the 11 year old. And only what I thought each could handle and then I let them ask the questions over time. Don't over load your 12 yr old give her time to digest it and ask you questions. Good luck!!

I would definately tell your Daughter. I come from a broken home and still have not gotten an honest answer from either of my Parents as to why they split. I feel that I deserve the truth and unfortunately will never get it even though I am now an adult with my own child. My ex had an affair and left one day out of the blue with a letter on the coffee table and I feel that when my Daughter is at an appropriate age she will need to know the truth. Honesty really is the best policy.