Recently, I had a mom who requested this as well. She wanted to participate with her son as if we were a play group, her words. I checked with licensing to see what would be needed and they told me that they needed to show up to date proof of a negative TB test and should not ever be left alone with children (other than their own). She did this and came for an hour and left. Since that first time, her child's attendance is sporadic, at best, and she doesn't stay in the class though she is always outside nearby and frequently picks up early.
So, my suggestion, if you agree to accept them, is have to it planned out that they would be able to lessen the time and have a predetermined date that the child would attend without the parents spending extra time besides drop-off and pick-up.