How to Build an Electronic Prototype?

You can get DECT phone modules which would do as your transceivers. A signal strength (squelch) circuit to determine how far away the sleepwalker is and some kind of alarm circuit on the receiver. Alternatively use PIR modules in a burglar alarm configuration to detect movement and how far the movement is from where the sleepwalker should be. You could get cute and make a transponder for the bracelet but you simply don't have the time.