A simple toggle, secured with a password would do it.

Yea, that's the thing - I don't think it would 'do' it for legislators. Like you mentioned - it's not really about protecting children, but also the only way to enforce a law like this would be to log or register devices to specific people or children. This would essentially just shift the point of verification from the individual website to the point of sale of the phone or tablet. Verifying the age is the part that necessitates identification - the only thing a hardware-locked strategy does is centralizes that verification to a governing body instead of individual websites, but it still associates individuals with specific devices.

I get why this might seem preferable, but the problem of online privacy still persists.