I just wanna throw my hat into the ring here.
First off, even though I have gone through several training courses in both offensive & defensive shooting techniques, to include several CQB courses, there is no way on gods green earth I would attempt a shot on a person holding a hostage, unless it was a contact shot-there is simply too many variables there, and I dont think I could honestly live with myself if I injured or killed an innocent person-particularly the child. I firearm used on the VEHICLE would be my option-it isnt jinking left or right, and is a rather large target. tires, radiator, and engine block. flat tires can be driven on, and if you hit the angle wrong, may not even puncture it. Or you can hit the rims (remember, at this point, your system likely dumped adrenaline, and your heart & breathing are accelerated). Better to shoot the front end-thats where all the vitals lie in the car. A cracked engine block will only go a short distance-same with a car with no coolant-once the engine is dead, thats it.
Now, put yourself in the abductors mind for a second, scary as it may be-he now KNOWS he has been identified (by you shooting at his car); what would YOU do? Ok, that may not be fair, but these people arent wired correctly. They likely will kill their captives, and possibly themselves, to avoid being caught. His WHOLE world just fell apart; this may not be the first abduction. This is a risk you take by actively engaging someone. Now, I am not saying DONT act, particularly if its your own child; I am just saying that, well, we arent superheroes-we let adrenaline take over, and the situation can potentially have a far worse ending.
Barring finding someone in my home, attempting to abduct my own kids, which is a whole different scenario, I would have to agree with Susan; Get as much info as possible-call 911 IMMEDITATELY....they record the call, and you can feed them the information. If needed, follow. If you were unnoticed, follow at a distance, feeding info. Let the police handle it-thats their job, not yours. If you arent familiar with the actors, yourself may be making a mistake-it could be, as someone stated, a parent picking up a child who is angry, and simply doesnt want to go home. again though, let the cops work that out.
I think that there are too many situations & variables to say with certainty what I would do. In my own home is a FAR different scenario than witnessing it on the street between strangers. In the first case, I would know that the situation isnt normal, and act appropriately. The second one, I would call 911, and follow, if possible-if not, I would feed them information actively as I am seeing it.
The human mind works in funny ways too...by recording what you are seeing, there isnt any guesswork. Most people wont remember the general details of the situation, simply because they are trying to focus on details. For instance, a "dark shirt" can be red, green, blue, black, grey. Thats kind of important if they pick him up immediately. Alone its nothing though. Which is why, IMHO, its best to call 911 IMMEDIATELY and report what youre seeing, before the person gets away. Cell phones are amazing technology; utilize it. There is no worrying about hitting someone else with stray rounds.