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1. Parents control what hardware the kids have access to. Perhaps not perfectly, but at least it is something the average person can see in meat-space and interact with without requiring special knowledge.
2. They buy devices for their kids that support a parental-lock feature, which (if enabled) causes the device to always send metadata outwards, such as HTTP headers. This metadata encodes some standard like "I'm a minor in Jurisdiction X" or even "I am currently approximately Y months old in jurisdiction X."
3. Online sites/services can comply simply by recognizing these signals and throwing up "denied" pages, without any kind of authentication and no need to build profiles of users or track activity.
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Sure, it's not perfect (kid borrows adult device, non-compliant sites, proxy sites that subvert the rstrictions) but it's better than nothing while avoiding massive rating-bureaucracies or forcing shitty backdoors into software. Also, the costs will tend to fall on the same people who want the feature.
Some pain-points will remain like whether to support a country where people under 28.75 years are not allowed to see ankle skin, but that's unavoidable complexity from the problem-domain--bad laws as opposed to bad software.