> 1) Given that it just says you're a "child", how does that work across jurisdictions where the adult age may not be 18?

So namespace it then. "I'm a child as defined by the $country_code government". It's no more of a challenge than what identity-based age verification already needs to do.

> 2) It seems like it could be abused by fingerprinters, ad services, and even hostile websites that want to show inappropriate content to children.

This is still strictly better than identify-based age verification. Hostile or illegal sites can already do this anyway. Adding a single boolean flag which a large proportion of users are expected to have set isn't adding any significant fingerprinting information.