I should point out that I have mentioned nothing about the OM, or anything bad about her other than she wants to leave.
Why are you lying to your children? They are adults; they should know the truth. You don't have to be nasty, or unnecessarily disparage their mother ("I still love her, very much, and don't want a divorce, but I've never lied to you and I felt you should know the truth.") But considering the emotional turmoil your children are going thru right now, they need to know that at least ONE parent is being honest with them.
