How about just prohibiting websites and apps from targeting behavioral ads at visitors across the board? Since kids will always lie about their age online, how will you be able to tell if the user is 18 or 8 if they use the same computer/phone/ or other device?
I'm pretty sure that the parents that want to be involved in what their child does online already are. The rest ether trust them, don't care, etc.
Don't let your kids go to youtube either! totally block that site as adverts and other adult content that is so not appropriate for young kids to view is always showing up. Even when watching Barbie or Polly cartoons there is this stupid video make up about a zombie all bloodied, not to mention the other content that is not related to children.