Surely some creative engineer could devise -- a 2-part device ... Already done; I had one and it worked great for keeping track of my kid while camping: Parent Pager.
(No, I don't own the company or stock.)
We just had a case in the Pittsburgh PA area, where a baby was left locked in a stadium parking lot in an SUV. Both parents thought the child was with the other and they went inside the stadium separately to enjoy the game. An off-duty policeman busted the window with a flashlight and got the baby out safely.
Inexplicably, neither parent will face charges.