She needs to figure out on her own what she is going to tell kids. I am working on what I will tell.
I encourage you to really think about this. I thought that I wouldn't want to present this is a mutual decision in front of my D, but changed my mind after speaking to a therapist who specializes in counseling children and after reading books talking about helping kids cope with divorce. I read that kids between the ages of 6 and 8 get a lot of their self-image from how they think their parents are perceived. So, if they think that you think their mom is "bad," then they own some of that themselves, too. We just told my D over the weekend. While we didn't say, "This was a mutual decision," we did say that sometimes things between grown-ups just don't work out, without placing blame on either parent.
If possible, I would encourage you to discuss with a counselor what you should say to the kids. One thing I've read repeatedly is that both parents should tell the kids together.