I know the textbook answer is not to get the kids in the middle and don't villanize your spouse in a D.
I have never mentioned my wife's affair with my ex-best friend to my kids. When my son asked me why we don't see OM, I just said we are busy and had a parting of ways.
If my wife left me for another man, I would tell them this is called adultery and it's wrong. They know it's wrong, they've had a strong community of faith around them for a long time.
If I were to D my wife, the only religious ground I have (which I take seriously) is adultery. If I chose that option, I would tell my children why I was divorcing her. Divorce is a public act with grave consequences. To tell the kids, "mommy and daddy just can't live together anymore" is vaguely truthful, but cruel. It gives them the impression that the world is random and chaotic. They live in moral universe and need to know it's rules. I would tell them that their mommy loves them and they should love her. It's OK to love people even if they do wrong things.
I don't think Frank needs to reveal much to them, anyway -- they know.
