I have to be honest with the kids about why we are divorcing, don't I?

Your first mistake is asking your lawyer this question, rather than a mental health professional. But since most people do ask their lawyers, the answer is no.

Reason #1: how you feel about your spouse has nothing to do with how the kids feel about him. They don't care if he makes eyes at other women whenever you go out together, and there is no reason why they should.

Reason #2: the kids can't divorce him, and probably don't want to. The kids love both of you, and if you force them to take sides they will blame you sooner or later.