My brother would put one on his car when my nephew was in the car but it was a magnet. His intention was that if he got in an accident, paramedics would know to look for and prioritize a child because they can be easy to miss in debris if they're unconscious, and they have a lot less time than an adult to administer life saving treatment. Tiny body, less blood, partially developed bones, etc. My understanding was that it wasn't the intended use, but it is a common use.