The place that is both technically possible, and desirable from a privacy protection view, to do this restriction is on the devices used by children, not on the entire internet.

Parent clicks "this device is for a child" and the device filters the internet and automatically uses safe search, etc, in a way that cannot be disabled without the parents pin. It can advertise its self to online services as a youth device (thats how google safe search already can work). The parent is in control and the child gets protected. Its not rocket science.