Reddit:

I imagine and wish Apple could implement a software download that allowed my old series zero to be handed down to my child with things like fitness and calendars still available. Maybe not even calendars.

I know the watch currently requires a phone but perhaps that could be tied to the “parent” device? Would anyone else be interested in this? How could it work?

I think there’s a germ of a great idea here. Maybe a dumbed down version of watchOS that let an older Apple Watch continue to serve and be a first-class citizen, able to communicate with the current version of iOS, but with a much smaller feature set. Still useful, an entry level device for kids, keeping old Apple Watches out of the waste stream.