I can remember getting this look when I had gotten into some sort of trouble of a less than serious nature that would have been something he had gotten caught at when a child. It is the "what-am-I-going-to-do-with-you-look." Being my father's daughter I got it a lot. It always made my mother absolutely livid.
In this particular instance I think the look was for a child at the guard rail. And his mother was close to being livid.