Because children shouldn't be the ones paying for the crime of having bad parents. Children should be safe regardless of what the parents do, because they're independent people and not property.
you can say that about a lot of things, but we do largely trust parents. but all these social media bans seem to stem from parents who claim to be helpless, when really they are lazy and the wider impact of these laws is massive. no technical way exists to do this without taking the privacy of everyone
A bit of empathy goes a long way. That child may be your nurse one day, taking care either of you or candy crush.