Don't see the point on cars, as I think it's true that no one would adjust their driving habits based on that sign. However, I think they make sense for people who carry their kids on a bike! Drivers, at least in my city, seem to be enraged that I exercise my legal right to ride my bike in the street, and that rage sometimes blinds them to the fact that there is a kid on the bike.
Studies show that drivers give more space to women bicyclists than men and also, less space to bicyclists wearing helmets, so it's clear that drivers do make some sort of risk assessment with cyclists at least. Therefore, it makes perfect sense to me that they'd also be more careful if they're aware that there is a kid on the bike.
And in my own experience, since I put one on the back of my baby bike seat I feel that drivers are a little better behaved.