The presumption that it's not a matter of the parents' prerogative whether to decide whether the child's access should be restricted or not -- and treating the parents as accountable to someone else's standards of what is or is not appropriate for their own children -- is itself objectionable.

What content is appropriate for children is properly up to their parents themselves, not to the government or to some nebulous concept of "society". If parent's choose not to set such a flag on their children's devices, then that means that they're choosing to allow their children to access content without restriction, and that's what defines what is OK for their children to access.