Create a standard header with a "Parental Advisory" PG kind of label. Then let the consumer decide what to do with it. The web is "content", and it is up to the parents to decide how to police their kid's content consumption (same as with music, video, etc).
Then we can start talking about Operating Systems, Browsers or Access points features allowing parents to block pages with specific PG ratings.
Lack of label should be equated to UNRATED, just like for movies. By _default_ the Internet should be for Adults who have their own right to choose what they see or don't see.
Let the OS / browser / etc WHITELIST things based on a claim something is safe for children, or rated in an academic context (E.G. Wikipedia / medical / etc).
Also interesting is this site: https://www.doesthedogdie.com/
Maybe this could go beyond parents. There are whole classes of image that I'd like to have to choose to see. E.g., if I'm eating breakfast and scrolling through Twitter, I don't need to see, say, mangled flesh in an accident photo. If there were standardized content tags, then parents could tell browsers to block outright and adults could make it click-to-view.