There's many reasons people, including children (who actually are people, contrary to how some on the thread seem to think of them as a different entity, probably because of their unfortunate lack of rights and agency..) don't want to be filmed without prior knowledge. It's not just about occasions of people in positions of power abusing that power, by a long shot, although lets be honest it's happen on far too many occasions.
I wouldn't want to be filmed without my knowledge at work - not because I'm doing anything wrong, although that shouldn't need defending. But because I'd rather know to save potential embarrassment. What if I choose a quiet corner, that I've not been made aware there are cameras covering, to quickly pull a wedgie out of my bum, that saves a 5 minute walk to the toilets, and a 5 minute walk back? Or to blow my nose, and check the tissue because I've not been well? Or quickly scratch an itch, at a well chosen moment, out of sight of others, or any manner of other utterly harmless, but socially embarrassing things we all do, but don't want to be recorded and seen by others? I would certainly be mindful of these things, if I'd been made aware I was being recorded.
CCTV is not the same, it's a different scenario entirely. Generally I don't care about people I'll never see/interact with, potentially never seeing the footage unless there was reason to view it, and even if they did see it, I wouldn't know them. Entirely different ball game to employers, or indeed teachers, whom I have to see again and interact with, seeing it.