"Baby on Board" is a terrible sticker for your car

https://piefed.social/post/754881

I always thought it was to let emergency responders know they’re there. I had stickers on my bedroom window as a kid for the same reason.

That’s an urban legend:

Unlike what is implied in the gruesome folklore that has come to be associated with these advisories, their purpose was not to alert rescue workers to the presence of babies at accident scenes; they were instead supplications to other drivers to exercise additional care, reminders that they shared the road with vehicles carrying children.

www.snopes.com/fact-check/baby-on-board/

Origin of 'Baby on Board' Signs

Rumor: The death of an infant in an auto accident led to the invention of 'Baby on Board' signs.

Snopes
That may not be their intended purpose, but it is something we take note of when responding to an MVA. “Who else was in the vehicle” is a pretty standard question to ascertain if someone has been ejected, or is currently entrapped in the inaccessible wreckage, but if we notice a ‘baby on board’, we always make sure to also ask something along the lines of “where is your baby”, just to be safe.