since my kid has been about 9 mo or so old, and definitely since he's been starting to be able to understand words and respond, he's known when he's been ding something wrong. even now at 2.5 years, if i'm on my macbook, and i want to show him something he is very delicate and doesn't touch the screen - because i taught him not to. even if he gets a little too excited, calm words like "Leo, easy." will sort him out.
i don't think most people give their kids the credit, respect and attention they deserve - thus, so many nanny 911 kids.