Largely agree with this, though I'll throw in that the OS should provide a signal as well. I know for sure that iOS and Windows both have family modes that work pretty well, I suspect Android does as well.

If my kid takes their tablet to grandma and grandpas I want the preferences and signals to carry forward, even when connected to a network at household that is nominally only adults.

These technologies don't need to be bullet proof to be effective and they don't need to send more information than "treat all requests from device as being from under 8/13/18." The ills these age verification efforts are trying to address (and they are real problems) are from excessive, not casual or incidental use. Yes, there will be many kids that get around any reasonable control, but just making it less convenient will reduce harm.

I have various content controls on at my house. I'm the admin, I can turn them off whenever I want to. I almost never do, because 1) the block reminds me I should probably shouldn't be going to whatever site I'm going to and 2) for the most part, my experience is better with the "restricted" search engines/youtube/social media.