For this incident I think nothing. I don't think it was the right way to approach the situation, but the child wasn't being physically hurt (in which case I would speak up). I wouldn't appreciate a parent commenting to me in that situation.
However, now I've read the OP more thoroughly (shouldn't bubhub and mark work :-p) I think I would ask the director to clarify their behaviour management philosophy/point me to a policy. If they seemed keen to know why then I might mention the incident I had seen.
I'm not really sure. You've got me thinking now. It's just so ingrained in me not to ask about other people's children, I guess I was looking from that perspective, if that makes sense.