Hardware -- What about cost, an Xbee at every node may do the trick but that will get expensive. Something like the Jeenodes may be good, they are essentially an Arduino and wireless comms on one small PCB.
Then each node has to have battery backup.
Software -- no idea, DIY I suppose.
If this was for a real project the safety aspects would be enormous, for a class exercise maybe you can just spell out what you would do for the real thing.
Rob