Shouldn't the verification be other way around? That is, you need to prove that you are a child. Then the site can present you more strictly filtered content. Parent can sign child's device on first boot, token stored in TPM so that it's hard to remove.

It's basically the same type of enforcement on sites, as they need to verify and filter content for children, or just block them. Most of the internet users are adults, why not make internet for adults by default.