The answer is actually fairly simple and easier to implement than ID collection.
My family for example, is all on apple products. So my kids are all under my family as kids in iCloud. With that information any web browser has more than enough information to just send a header saying “No porn or non-age suitable things to this device, please” as a header
It mostly respects privacy while putting the onus of not serving non age appropriate things to the service provider that can serve the non age appropriate thing.
Instead we are going for collecting IDs because some idiots want to strip anonymity and even bigger idiots don’t know how to setup devices for their children