I love the shot. When that happens to me, I go up the the parent (or person if it wasn't a child) and tell them I'm a photography student (or photographer in your case) and thought it was a great moment to capture and that if they give me their email address, I'd be glad to send them a copy of the finished shot for them to keep (I'll eventually make business cards with my email on there to hand out so they can write to me and I respond with the image). I used to ask before shooting but 1) asking them makes them think that they have the right to say no (ha!) 2) once they know they're going to be photographed, they either smile, tense up, or hide their face if they said no.
I think the insult was worth it, though. Great result! :thumbup: