as for the sign being a warning to rescuers in the event of an accident or fire, my concern is that some people may not actually have kids in the car at the time of an accident. if a rescuer didn't find any kids in the car and saw that sign, he may be tempted to put himself in harm's way by going back and continuing to look for the nonexistant child. i think it's standard practice for rescue workers to look pretty thoroughly for any passengers, with or without a sign.
There was a debate about the sign on one of my English canine fancier websites I frequent several years ago. Apparently, they were originally designed to inform emergency personnel to look for children who possibly had been ejected from vehicles during accidents.
The signs came out in either Australia or UK in the late 1980s or early 1990s. When they arrived in the US, everyone thought they were cute or 'warning other drivers to behave'...
Now that (hopefully), car seat usage is up, there's not as much concern about locating lost children, but the signs were never advertised for such use over here, so they became a vanity issue, imo.
Personally, I think they're kind of worthless and are aimed at those parents who like bumper stickers saying "My child is an honor student at ______"
I've seen Australian vendors at the military bazaars with all kind of 'warning' signs. Some are very cute, especially dealing with wallabys & sharks