My explanation to my sons - "Papa has a different way of relating to people and the world than we do. He's a very unhappy person, and doesn't know how or isn't interested in, making himself happier. Because of this, he tends to be super-critical, and tries to manipulate people, particularly those he is close to and comfortable with, in ways he thinks will provide happiness for him. This is unacceptable behavior and can cause a lot of hurt to those he belittles or manipulates, and since it's much easier for him to "use" people who have little or no defenses, or are not in a position to use those defenses, I've made the decision that I won't expose you to him, if at all possible. Of course, when you're older and present less of a "target" to him, I won't forbid contact between you and papa, nor will I encourage it."
My sons are teenagers, and since they (unfortunately) are very aware of the dynamics between Ndad and me, I felt I could be open with them. With younger children, that approach may be too direct.
Good luck, nihil. These days, children get burdened by so much at such a young age anyway; IMHO, the longer (within reason, of course) they can remain "children", the better...
bobbie