The baby scenario happened sometime back and was highlighted on America's Most Wanted, only nobody saw the baby go out and inside the confines of the car seat, the baby survived being discarded at speed and later simply died of exposure.
After you gently collect the baby, making sure not to exacerbate any injuries that may have occurred in the fall (sorry, no high speed snatches off the pavement, wingnutz), you then get the baby to a hospital. You do not give chase. That is not to say you don't continue in the same direction as the bad guy, if that is the closest direction of help, but you don't actually give chase.
Heck, if the car is an old beater POS as described, there is some good likelihood that I would end up passing the guy on the way to the hospital.
When somebody else's life is in your hands and their survival depends on you, you don't perform any sort of vigilante after the fact justice no matter how good it might feel to you. I have 2 kids that ride in car seats. I know how I would feel. Even so, the ONLY priority is getting medical attention for the kid. Unless you are an MD, then more than likely you won't have the training necessary to determine what trauma the kid may have suffered even if the kid itself did not actually hit the pavement and shows no immediately apparent signs of trauma. Such violent episodes can result in shaken baby syndrome where the brain gets bruised and can result in swelling of the brain that can be fatal if untreated.
Better to save the kid and let the bad guy get away than to get the bad guy and let the kid get away (dead).
