Options for short range distance determination between two objects
This idea might be patented, so it might not be suitable for a commercial project, but you can actually measure the position and orientation of one electronic device relative to another, with reasonable accuracy, using magnetic fields. This is how Polhemus and Ascension trackers work. They are used in VR motion tracking, and in surgery for tracking the position of surgical instruments during operations.



The basic concept is to have one set of coils transmitting, and another receiving. The transmitter coils emit audio frequency alternating magnetic fields, and the receiver coils then measure the amplitude of the fields in the three receiver coils. 

There is some code available online for doing these calculations. You might also take a look at the guy's project page: Open source electromagnetic trackers using OpenIGTLink.

This may not be quite the system you're looking for, as it's fairly complex, and is giving you much more info than you wanted. However, a simpler algorithm could be used which just gave you distance.

A company called Sixense make a gaming controller with a 6DOF sensor in it. I don't know how easy it would be to integrate this technology into your project though.

Update:

Now that I know what your application is, I have been thinking of a very similar application. My suggestion would be this:

Use the three orthogonal coils approach. Both the mother and child have a set of coils. The child would be the transmitter. Every few seconds, the child module would transmit an acoustic frequency magnetic field on each coil in turn. The mother module would measure the amplitude of the voltage induced in its coils. If the amplitude was too low, or if no signal was heard for more than a few seconds, then the alarm sounds.