I have seen many times, and as recenntly as a month ago, a unit that is used to tell when a small child has wandered away from its mother. It consists of a small wrist unit, and a slightly larger wrist unit for the mother When the 2 are seperated by a specific distance (I dont know the distance, but 20 feet rings a bell) the mothers unit squeels.
I remember seeing the unit at Radio Shack (here in Canada, relabelled as Circuit City) for around 30 bucks.
Good luck.