FRS works fine in cities, with a range of a few blocks. That's plenty for many purposes and completely inadequate for many others. Ham repeaters have much more range. In a real emergency, ham communications can switch over to a message handling mode with emergency traffic (medical stuff etc., where someone will die if the message doesn't get through) taking priority. Stuff like coordinating family movements (what it sounds like you have in mind) would be lower priority.

I've been thinking for a while that the ham world needs a good, portable, text-only communications device that would use very low bandwidth (up to several minutes to send a 100 character text message). Because of the low data rate, it could cover long distances with very little xmit power. I'd been thinking of it more as a cheap alternative to a satellite phone for remote locations, but it could also be good in those urban situations.