Let me ask you something. If your Ex agreed at mediation that he/she would not longer date the felon or bring him around your child, would you trust him to keep his promise? I didn't think so.
So mediation is not going to solve your problem, is it? What you need to do is take the gloves off and fight it out before a judge. You need to get full custody and get your Ex to have only supervised visitation. Not only is drug use around the child terribly harmful to him/her, but so it witnessing domestic violence and the peculiar dynamics that go with it. (Scared, immediate compliance with all the abuser's orders is also harmful to a child, who is learning how men and women relate to each other).
In my view you are just going to be going through the motions at the mediation, but in the meanwhile you should be gathering proof of your points about them, even if it requires investigator surveillance.
You definitely need an attorney to fight this out. The alternative is to accept that your son will learn to abuse women or your daughter will learn to admire men who do it.