A agree that your hypothetical situation could be problematic.
But I’d submit that preventing parents from observing the virtual classroom activity is addressing the wrong end of the “problem.”
The source of the problem isn’t the parent observing the classroom- it’s the actions of the parent.
So, rather than create a policy that makes it appear that there’s something to hide in the classroom lessons, why not create a policy (with a signed acknowledgment) that it is inappropriate for a parent of one student to contact other students? That address preventing inappropriate behavior, rather than appearing to shield the classroom from “prying eyes”.
Because my reaction to being told that I can’t monitor the virtual teaching of my children would be very similar to @RJ.