Again, without knowing all the facts, it is all pure speculation but if the parents did try to close/obstruct the monitor themselves, were told not to, continued to the point where the FA couldn't leave the area without the parent trying to do what they were told not to do as soon as the FA was out of range, then they most definitely interfered with the duties of the cabin crew. Even if the FA suspected that if he/she left that the parent would immediately tamper and possibly damage the equipment, then that FA and possibly others is now completely focused on one issue and not able to do their job.

I suspect it is kind of like that. Like the guy who is told to turn off his cell, hides it when the FA is there and brings is out as soon as the FA walks by (Alec Baldwin anyone?). I can see it happening where these parents waited for the FA to walk away and then tried to close/obstruct the monitor after being told not to. If that isn't disobeying the flight crew, then I don't know what is. If that is what happened then precedent shows that planes have been diverted for very similar situations. And I think if that was the case, then what else was the captain to do? Just let these people do whatever they want as they obviously weren't going to listen to the FA?