Having formerly been a single parent, parental abduction has been something I've considered at length. At the time, I was employed first in private security, then with a sheriff's department. I at least knew where the threat might come from, but not when.
Any reasonable parent shouldn't mind someone stepping in to politely attempt to look after the best interests of their child. I sure wouldn't. If the man in the scenario given reacts in a hostile manner (e.g., tells you to mind your own *@# business), you are still in the right to talk to the child to make sure that all is well. Child abduction is too common in this country for any decent person to not intervene.