It'd be more useful than DNT because there would be legal teeth on the side of the sender and receiver of the signal. It'd be more useful than the yes/no gates because an operating system could choose to allow the creation of child accounts.

I.e. it would be a standardization of parental controls with added responsibility on sites/apps to self-determine if they should be blocked or limit functionality, rather than relying on big white/blacklists. Basically an infrastructure upgrade, rather than relying on a patchwork of competing private solutions to parental controls and age checks. The hope is also that a system like this would remove concerned parents from the list of supporters for pervasive mass surveillance and age scans. If they feel like you'd need to be a moron to miss the "This is a child device" button while setting up their kid's phone and laptop, and it's broadly understood that just pressing that button locks down what the device can access pretty effectively, that puts and damper on the FUD surrounding their child's internet usage.