Young mother with her boy

So he's sitting on the underground in London, and there's a young mother with her son. He's around 4 or 5. Apparently this kid is being kind of annoying, making noise, running around etc. Eventually the mother loses her cool and slaps the boy on the arm quite loudly. There's a moment of silence, most of the people on the carriage were watching this kid, so they all saw her do it.
For a second it looks like the kid is about to start bawling, but instead he turns, looks his mother straight in the eye and says; "Are you proud of yourself?"