I see one use case : we'll be travelling all summer across Europe. One family on a tandem and two regular bikes. There's a pre-teen that will, most likely, ride ahead, and sometimes behind. I can easily imagine scenarios where we will lose visual contact and might appreciate to see where everybody is.

I believe that Strava offers the option. I am almost certain that oruxmaps offers realtime shared tracking.

This would require Smartphones and considerable battery power for everyone involved, unless the app you have in mind could be activated on a need to know basis.

You may want to get in touch with oruxmaps. If I recall their app requests permission to access SMS, which can be sent with the lat long data. This might be how unsuspecting riders may be warned that something is amiss (the distanced following rider gets a flat, unnoticed by the leader)