This is actually a great subject (tangent) that stems off the original question. Interjecting yourself into a situation that you don't have an intimate understanding of is very dicey.

A popular scenario thrown around in concealed carry discussions revolves around encountering two men in a fight. John Doe intervenes only to find out the the opposing party was a plain clothes police officer. Shooting a police officer (in plain clothes or not) typically does not look good on a resume.

Specifically, in the scenario initially posed by the OP, in my local I would be justified in using deadly force to prevent the abduction of one of my kids. That does NOT mean that I'm going to shoot someone pulling a screaming kid into a car even under suspicious circumstances.

If I were to observe what appeared to be an abduction in progress I would call 911 and do everything in my power to make myself known & remain in contact with the actor(s) until police arrive. If an abduction is really playing out, hopefully the perpetrator will release the kid and run. At the very least they now have to deal with a physically capable, armed adult male instead of continuing their attack on the child. If the scenario playing out is benign then the actor(s) can explain it to the police when they arrive.