If Keith doesn't trust his mother, it will reduce his confidence in the security of his future. One thing children of divorce desperately need their confidence that they will be well cared for. Keith's relationship with his mother creates the foundation of his future relationships with women, and it is important that the foundation be strong. As you know, children often blame themselves for their parents' disputes. If Martha criticizes you to Keith but you fail to return the attack, Keith's respect for you will grow.
Children are remarkably intuitive about fair treatment.
Judges greatly disfavor parents who criticize their former partners in the presence of their children. This factor is highly relevant to the judge's determination of custody and visitation issues. Don't argue with Martha in Keith's presence. Be careful not to argue during visiting exchanges. Don't ask Keith to deliver messages, money, or anything else to Martha.