What I'd do:
I would QUICKLY do the following, if I believed I was witnessing a forcible kidnapping:
1.) draw to a safe indoor ready-right carry,
(yeah, I know, dramatic SWAT picture, but it was the first one I found at the indoor ready; Weapon on
safe,
trigger finger straight and
off the trigger, muzzle down.
2.) move to a covered and concealed position, intersecting the perpetrators
3.) give strong verbal commands to "get down on the ground now!"
It would be important to keep your weapon at the ready, in case you are engaged by the perpetrators, and be aware that law enforcement / plain clothes officers could be nearby. I would keep giving commands to get down, then I would secure the girl if possible. The moment law enforcement arrives, I'd drop my weapon and show hands, following instructions from there, to avoid any potential confusion.
If the perps continue abducting the girl, I'd fire center mass repeatedly to stop them. I feel that once they get out of the building the chances of stopping them are close to zero, and the chances of that girl surviving are less.
In this case, deadly force would likely be justified, since a reasonable person would fear for the life of the victim. The actions of the perpetrators put the girls life in imminent and demonstrable danger of death. As was said earlier, the victims of these kinds of crimes usually end up dead within 24 to 48 hours, floating in a shallow muddy ditch. I couldn't live with myself if I had the means to stop a kidnapping, and didn't- and it ended with a dead little girl.
You aren't law enforcement, and its not your "duty", but I've got a conscience, and I'm not going to live with something like that for the rest of my life. Weapon at the ready, professionalism with regards to muzzle position, cover and concealment, and strong authoritative commands are the only thing one can do.