Any parent who is concerned for their child's well being and allows anyone else to watch their child without any discipline is a moron and is jeopardizing the safety of their child.
Any judge who allows such an agreement to be signed is a moron and jeopardizing the safety of a child and should not be in their position. What does the judge suggest the SP should do if the SC if the child is doing something that puts them in harms way? Turn a blind eye and hope alls well that ends well? What does the biological parent suggest in this situation?
I hate to sound nasty, but I do not believe this happens. I do not sit in court but have enough experience to know custody agreements do not work that way. I can see a step parent not being allowed alone with a child, and I can see strict terms in place as to what is allowed, but not one where a child is allowed to spend time alone in the complete care of someone but not allowed any terms to keep their child safe(which in many cases is what discipline is).
Also by the ex agreeing that a SP can be alone/watch your child they are choosing them. They have every right to say that person cannot be in that position with their child, but by saying yes they are choosing them to be a caregiver of their child just like any coach/teacher situation. They can't tell their ex to find a new spouse, but they can say nope I don't feel safe with our child being left alone with that new spouse.