Perhaps late to solve this globally but parents can still install parental control software if they so desire and can still intervene locally to prevent sharing data with 3rd parties. At worst this means small children might not get to visit social media and other assorted sites and I am fine with that. I think a number of parents would be fine with that as well.
Sites can voluntarily label as some do. It just means that parental controls would have to default to blocking everything until approved and while sub-optimal maybe that's what people will have to do in order to avoid the evil pattern of sharing data with all the websites that will ultimately leak, or "leak", be sold, stolen, etc... Good parents will not participate in the evil patterns of sharing their children's personally identifiable information.
When the PII of children is ultimately shared with evil people the children once adults will resent their parents for not protecting them.
- To all parents here, your children have no idea what risks are out there including devious companies that want their data. They will one day be adults if all goes well. Protect your children as corporations and governments will not. They will thank you when they find out all their friends data was shared, leaked or otherwise abused forever.