I'm not really a fan of the solution (something more along the lines of the v-chip where commercial sites in the US are required to identify their content categories in a standard way so browsers could be configured to block it seems like a better solution), and sites like pornhub are straightforward enough to include in blocking software, but currently sites like Reddit present a problem. Looking at recent dumps[0], it looks like ~40% of posts on Reddit are marked nsfw, but those posts are on the same domain as things like /r/teenagers and /r/roblox. Without a MITM proxy, parents can't really block the porn half of the site unless they block the whole thing.