Also the device ID's are already shared internally in some fashion across an iCloud family, as my key tracker doesn't make my wife's phone detect an unknown tag and vice versa.
Ideally you should be able to share beyond the family, IE I let my parents borrow the car, I should be able to "loan" the tag to their account (And I guess choose whether I can see tracking or not.) But if we don't have family plans we obviously won't get to this point any time soon.