Or this should be done at point of sale, like we do with all controlled substances.

We don't sell bottles containing alcohol and then expect to filter the alcohol out if the child wants to drink from it. We have two different bottles: alcoholic bottles and non-alcoholic bottles. If you are a child, you cannot purchase the former.

Stop selling unrestricted computing devices to children. Require a person to be 18+ to purchase an unrestricted internet device. Make it clear that unrestricted internet access, like alcohol and nicotine (and the list goes on) is harmful to children. That resolves 90% of the problem.

And lets be fair, the problem isn't the children. Children want what all their peers have. The problem isn't their peers. The problem is the parents. Give the spineless parents a simpler way to say no to their children, and the overall problem goes away.