> (Parents should opt their kids phones into “kid mode” and this would block age-sensitive content. The law just needs to mandate that this mode is respected by sites/apps.)

Good kid mode[0].

[0] https://www.lego.com/en-gb/product/retro-telephone-31174

Adding a kids mode to *all* sites seems like a huge investment to most of the tech industry. I predict most would just NGINX-block users with the kid header.