It would be possible to make a website which proxies other sites, but strips this header, right (maybe with some added ads)?
If so I would expect such sites to appear, and the only way to secure a child device is to have a whitelist of webpages (to avoid the proxies), putting us back close to where we are today.
Such sites would be illegal if not sharing the header back from the source website and be banned as much as adult websites incorrectly setting this header. It’s not a real problem.