I used to go to stage doors after viewing a theatre performance, and I've been in situations where the security guard at the door specifically asked that no pictures be taken of so and-so, and if he saw any such pictures being taken, he would personally delete them, and check that person's phone at the end. He also said that if those rules weren't acceptable, people were free to leave. Something similar can be done at any venue that Catherine, William and their children choose to visit on a family outing. People can be let inside in small groups to help crowd control, and to make it easier to keep an eye on the photographing. If people don't want their phone/camera checked, they can be mindful of what they photograph, delete an accidemtal/intentional snapshot of the royals, do without photographs, or vacate the premises and come back another time if the rules are not to their liking. The RPOs don't even have to get involved.