I have a 2.5 y/o boy. He has NO problem understanding what is acceptable and what is not, PROVIDED he had exposure to at least one (or maybe two...) previous examples of what is not acceptable behaviour. Many times he heads down on the path of *unacceptableness* surely fully aware that what he is about to do is wrong. One can usually tell just by the facial expression, or by his/her desire to move away from the radar screen to experiment the dark side. Else, you can tell they have no idea they are about to break rules just because they are following their curiosity and minds and are not aware of any previous reprimend/consequences. It's very, very easy to tell one from another.
The * help" cleaning the window is a perfect example of a positive mistake.
I suggest you watch the Brit. TV series "Supper Nanny", there is some pretty good advice in there.
Incidentally, I am only answering this topic because of Foucault's "Discipline & Punishment" classic...., which caught my eye. I am a lurker.
http://web.ics.purdue.edu/~felluga/punish.html
