I agreeThere is no possible way I could not intervene in something like that. Calling the cops is not "intervening". Once they leave the scene then the odds of getting them back safely go WAY down.
If it's the girls parent I hope they would care about their kid enough to be thankful someone is willing to probably save their life if they were abducted (I know that's wishful thinking & they would possibly act like a JA ). If their not the kids parent I really don't care what they think.
Either way you would be completely justified in using deadly force to stop that person from leaving with the kid as soon as you heard "I don't know you". That would be a reasonable belief that someone was in threat of serious bodily harm at that point.
I would not NECESSARILY jump immediately to DF but depending on how the person responded to any questions determines my next steps.