The child, or in the instance I'd like to try- cell phone, wallet, etc. would have the receiver. So whenever the receiver reaches a certain distance you would be alerted. Thanks for the reply davenn, that's very helpful for pointing me in the right direction.
First, I know nothing at all about this topic, which is why I'm looking for help. I'm interested in building a radio transmitter/receiver like a child locator at home. The difference would be I'd like the receiver to beep rather than the transmitter, if that's even the correct language to use...