I agree with the parent here. If the device is designated a kids device by some means, websites can ensure no adult content gets transmitted and their data isn't mined for ads.
Why is anyone's data mined for ads? If you're going to make a kids mode flag that denies data mining effort, then just fucking make the don't mine me switch and open it to everyone.
Well, currently ads can be targeted using user agents. This is analogous to that sort of attack, although less malicious. There are bad people who would target children if they could, so it does seem to fit with what you're asking. We create the means, and they will exploit it.
> I’m not sure that making kids identifiable as such to any random website is a good idea.
This is what any of the age verification proposals do, isn't it? You know who the kids are and who the adults are regardless of whether it's because you have the adults assert they're adults or the kids assert they're kids.