There is safety in numbers, so the children should remain together. That's the only way supervision will work, anyway. They have to be together.
Give the older one a cell phone and have the child keep it turned on. If any abusive behavior starts, the child can call 911. Do not call the children on the phone or allow them to use it for anything but calling the police for help.
I know it will be difficult, but do not scare these children with questions after visitation or 'drills' before visitation.
Unfortunately, the judge has chosen to ignore proven statistics that sexual abusers and sex addicts are the least likely to ever achieve any degree of recovery. Hopefully, the father and his wife are sufficiently fearful of the consequences.
Have the children routinely seen by their personal physician and ask the physician to be particularly observant for any signs of sexual abuse. Do the latter in complete confidence with the physician and do not let the children know that you and the physician have spoken.