For the context of a prototypical nuclear family unit it all seems pretty straightforward, but once you introduce extended families, blended families, nannies, or abusive parents or partners you suddenly get a lot of complications and a level of granularity with permissions that would get pretty intricate.

For the CarPlay use case at least some kind of ad hoc “party” entity that all the devices flow into might be interesting. I’m thinking about how with the original StarCraft game one disc had a license for up to 8 instances of the game to play via LAN so you could have a single license key allow a whole LAN party. Some system like that where the auth flows through a “primary” account but everyone in the “party” contributes their own entitlements to it and can provide input.