[cityroasted]

I was explained that the primary purpose is so emergency responders know to look in the event of a crash.

[ThinkingGuy]

I once asked a first responder (firefighter) if they actually pay any attention to those signs when responding to a crash. They don't.

[brlewis]

A professional first responder is going to look in the back seat regardless. A first-on-scene good samaritan might not.

On the other hand, I've had 3 babies and never put up such a sign, despite feeling very protective of them, so my intuition may be signalling some other reason why such signs are useless.