Locator devices for lost toddlers ?

Hi We had a scary experience at the mall tonight when my wife and I temporarily lost our 3yr old in a store. He slipped our view when we were tending to our 2month old. Fortunately we found him when he was hiding behind some clothes racks. Thus we are interested in buying some child locator device to attach to his shoe or other part so we this doesn't happen in the future. Any advice on a product to buy? A google search led me to some devices from Brickhouse Security? Any thoughts on these or other products would be very much appreciated!