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How does sending a header with extra information make the world a worse place for everyone else?

It still requires arbitration of every digitally communicated thought, whether it's age appropriate and to what degree. It's mass thought policing, as well as trying to enforce a cookiecutter morality on every person.

I get the desire to enforce thoughts on someone else. But fight it, please. At least let people be free in their own mind.

Especially since the tools to achieve your stated goal, protection of your child(ren), are already available. So you can perform your censorship desire in the confinement of your family bubble. No need for collateral damage.