Because I am 73 years old, I hopefully would be smart of enough to take a picture of this abductor with my smart phone, or at least give a good description of the person to the police. I doubt at this age I would attack this person, and depending upon the circumstances would care more about getting medical treatment for my Granddaughter.
Since I live in America, I would have the person arrested, what do you want me to do, to take out my weapon and shoot the mother fucker. My crew once asked me a question about a scenario where my son killed someone, and they asked me what I would do. Would I protect him. I politely told them that I would advise my son to turn himself in to the police, and that he would need a lawyer to defend himself.
Your question is redundant, why would I ever hit the guy, for hitting my Granddaughter. That makes no sense. I would see that he was arrested and charged with battery.