Parents have the tools already here in the UK.

ISPs come with adult content disabled by default and someone has to opt in to it. Every major OS (Windows, Mac, iOS, android) ships with device level parental controls. Games consoles enforce these based on birth date. ISPs here also provide free network level filtering on top of that. All of this only matters if the parents don’t bypass them when asked.

If a kid is determined enough to get past Apple family controls and the network level filtering on their home network, they’ll have a VPN from a dodgy source in 15 minutes. The solution is to use the tools that are there right now, or accept that age verification is coming for everything.