There's a better network layer solution than trying to block visitors at every host or at the endpoint devices' OS/application layer. I know because a company in my country is already testing such a thing in private beta. A router firmware that provides two wifi endpoints per home, one for adults and one for minors. You only share the minors' password with your children. The firmware maintains a whitelist of sites (it periodically downloads from a central server, I think). There is nothing to be installed on endpoint device, and no gatekeeping is necessary at the host. Apparently this can be extended to mobile networks as well, but I haven't looked at it in detail.