So I agree that operating systems and device developers should not be liable. That's putting a burden on an unrelated party and a bad solution that does possibly lead to locked down computing. I meant that liability should lie with service providers. e.g. porn distributors. The people actually dealing in the restricted item. As a role of thumb, we shouldn't make their externalities other people's problems (assuming we agree that their product being given to children is a problem externality).

What if all the useful apps refuse to run on the childproof operating system?