For one thing, it's fairly uncommon for children to purchase operating systems. As long as there is one major operating system with age verification, parents (or teachers) who want software restrictions on their children can simply provide that one. The existence of operating systems without age verification does not actually create a problem as long as the parents are at least somewhat aware of what is installed at device level on their child's computer, which is an awful lot easier than policing every single webpage the kid visits.

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).