I've thought about something like this. I like the strategy where only two people need to keep the app working, they could be sure they have adequate battery power ahead of time. What you basically talking about is location based service. Google had the latitude app, but shut it down for some reason. Try Life 360. I haven't used it, but it seems like it would work. it works on both iphones and android. I'd like to know how it works out for you.

here's a recent review of location aware apps:
http://www.tomsguide.com/us/best-loc...view-2405.html