Our modern lives have been shaped by technological breakthroughs and scientific discoveries that make our lives better. Often, the "making life better" part comes a long time before the "whoops- that's actually very dangerous" part. The automobile changed family life forever, as the large sedans of the past could haul all the kids at once! They could even be comfortably rocked to sleep in a baby hammock suspended across the back seat! The ad touts how this would keep your baby safe in the car, meaning something like safe from rolling down the window and jumping out. However, in the event of a collision, it could easily become a slingshot.
But the car hammock is just one of many products that went to market without someone trying to imagine what could possibly go wrong. Read about five of those dangerous items especially designed for babies, or rather, especially designed for the convenience of parents to the detriment of the child, at Cracked.
(I think that last is a form of metonymy - if so, my 11th grade English teacher would be pleased.)
When we got bigger, my sister and I would stuff the footwells and and nap laying flat. W/o seat belts, of course.