for about $35 bucks you can get "Child Guard"
here's the description:
Worn by the child, the cartoon animal-shaped Child Guard transmitter sends a constant signal to the receiver, which is held by the adult. The signal is adjustable -- from a distance of 6 to 30 feet, so when the child goes beyond the set distance, the adult's receiver starts to beep, letting you know that the child is starting to wander off.
Frequencies adjust themselves automatically to alert the proper receiver if more than one adult is in the same area using the Child Guard monitoring system.
Simple and easy to use compact wireless design.
Child friendly design -- transmitter is hidden in a plastic toy.
Has a rotating dial that you can adjust anywhere between 6 and 30 feet.
Transmitter can be attached by buckle, to clothes, or worn as a necklace.
Suitable for indoor and outdoor use.
Lithium batteries included.
Easy to read instructions.
Retail Price $35.00
If you don't mind the cartoonish Xmiter it comes in green, pink or yellow. just throw it in you laptop bag its a cheap solution.
Otherwise your looking for something like the Caveo which is $99
Here's the link for Child Guard:http://www.mainstreamsecurity.com/products.asp?id=14