One of the things just about every blog touching on self-defense, and every trainer having something to do with it, stresses is awareness. Of what's going on around you, who's around you and so forth.
Go read this at Xavier's. Being aware, and quickly figuring what to do, at the least kept them from being robbed: at most, injured and maybe killed.
I'd guess they saw a man with a kid and the magic box, and thought he'd be too busy with the kid to notice them 'till too late. I'll throw out a thought: a man, even a good-sized one, who's too preoccupied to take notice of what's around him will be seen as a better victim than a smaller woman who's seeing what's going on.
* I think it was Churchill who said something about democracy being two wolves discussing lunch, and a well-armed sheep arguing the point.
Correction: it was two wolves and a sheep discussing lunch; a republic is a well-armed sheep disputing the menu.