Our child is thirteen, and the “share airtag with” is not available.
Therefore the airtag attached to her set of keys is only useful to her.
The main point of getting airtags for each family member was to be able to find each other when the case arises: father traveling, and being able to be spotted in case something happens. Likewise for mom. If youngsters are missing for a while, it would be reassuring to know location, not to mention a school trip.
If we want to get into mud regarding privacy, freedom, etc. keep in mind that any member of the family could willfully refuse owning, or, even better, could refuse to share. I even thought the child sharing feature could have been conditioned by Screentime, wich is a pretty good means of controlling against misbehavior, and time management.
Sharing the airtag feature is a small step forward, but so far, for our family seems to work fine as a reminder we left the keys behind, not much more, and how to spot them.