I can't stand these things - oh look at me, I've had a child, aren't I wonderful, but please be careful, don't you dare crash in to me.
My closest friend has been in the fire service for 20 odd years, and he reckons these signs have absolutely nothing to do with alerting the emergency services as a thourough search is always made anyway.
Can't stand the derivatives either - Little Princess on Board, Cheeky Monkey on Board et al.
We were given a couple when our first child came along and I steadfastly refused to have them in the car.
Also, have you noticed the cars which have "Babe On Board" signs are invariably driven by women that could be described as a lot of things, but Babe would not certainly not be one of them?