Hopefully, I never encounter that situation. Nobody truly knows how such a scenario will play-out or how they will react beforehand.
What I would hope to do depends on the situation---if it occurred in a public place, such as a shopping mall, I would first call 911 and then confront the person directly.
If there were no people around, or say, if it were dark out, I would call 911 and then follow the person, incognito, so as not to arouse his suspicion that anyone was on-to-him. If he gets suspicious, he will make a desperate act, such as running or ducking into an alley, etc. at which time, I would do everything in my power to keep pace or even apprehend him, myself. Hopefully, the police wouldn't be far behind.
If he were heading towards an automobile, I would confront him before he had a chance to get-in, and weigh the situation based on his answers, his demeanor; the child's demeanor, etc.
If they were already in a car, I would get the make and model of the car, the tag number, etc. and call 911 and follow it, if possible.
The most important item on my agenda would be to free the child, regardless of whether or not the kidnapper was caught.
And whether or not the child was actually being kidnapped can always be determined later. The important thing is that the child is safe, and the suspect who was mistaken for a kidnapper should understand and appreciate the precautionary measures. I certainly would, if it were my child.