In Japan if you are afraid of losing your child in the shopping mall you can buy a “Lost QR Code Pendant” to hang around their neck (see bad marketing video below). Made from slate, weighing 10 grams and complete with kangaroo leather case it costs $42. The supplier recommends that the QR Code resolves to the parent’s contact phone number although if I was a kid that had to wear one I would prefer it said “please buy me ice cream until my daddy finds me”.
Nice idea but at that price it’s expensive.