At first, it seems innocuous: baby on board! Please be a little more careful driving near this vehicle.
But what does this imply? Shouldn't we be driving carefully enough to preserve life and health, regardless of who is in the next car over? Does the absence of a "baby on board" sticker imply "don't worry about us as much! Feel free to be more aggressive" ? I'd hope not. Besides, babies aren't the only people who are more fragile than an assumed strong, healthy adult. Cars everywhere have plenty of elderly people and people with various conditions who would be in more serious danger in a car crash. I'd go so far as to say everyone on the road is somewhere on a spectrum between "peak human physical and mental condition" and "extraordinarily fragile", and we cannot know or judge where each person is at. We should be driving carefully out of an abundance of caution no matter what.
Maybe the idea is based on assumptions about the longer potential remaining lifespan of a baby, compared to an adult? A kind of utilitarian argument about reducing harm by preserving more years of life? If that sounds a bit familiar, it is part of the concept of Quality-Adjusted Life Years (QALYs), which is an integral concept in Effective Altruism, among other uses. I won't go into the details of Effective Altruism, but let's just say it's a pretty terrible system for deciding where to spend resources that glorifies the ability of rich people to do philanthropy and provides support for some racist and eugenicist outcomes. In any event, it certainly doesn't make sense for every driver on the road to be making some kind of QALY analysis of their neighbors on the road at every point of the journey.
I believe that all life is precious. I make an effort to drive that way, too.
P.S. the best interventions to make driving safer for everyone are systemic and infrastructural: crash-compatible vehicles, available alternatives to driving, slower speeds, modal filters, etc. I don't blame baby on board sticker havers for the absence of these things.
Well yeah. If you're at fault in an accident and you killed a baby, the jury is not gonna be happy with you. If you killed a non-baby maybe they'll let it slide depending on circumstances.