I've never really figured why these pieces of plastic exist. They can be seen proudly (or rather unashamedly?) displayed in the rear windows of cars driven by new parents. But why?
I assume they are intended as a request to other drivers to take extra care around the car displaying the announcement. In practice, I can only imagine they serve to agitate said other drivers, thereby increasing the likelihood of being rammed from behind (as it were).
If you're a new parent thinking of buying a 'baby on board!' sign, save your money. As that little one gets older, you're going to need every penny you can save.