Children wireless electronic 'virtual leash'

I’m looking for someone to design a children’s electronic “virtual leash”, basically a transmitter/receiver, both of which beep when they are a certain distance (circa 30 feet) from each other. The product is meant to be a safety device, to be worn by both parent/child, which alerts the parent when their child wanders beyond a distance from them considered safe, primarily in crowded areas. I’d like the transmitter/receivers to be placed on a Velcro strap, which can be used as a wrist band, or strapped onto any object (purse, stroller, etc). The look of the transmitter can be discussed in detail later, however, I’d like a very simple non-descript design. For products similar see the “Child Guard panda” and “Angel Alert Child Distance Monitor “.