[Faceman]

I think the last posts made something very clear.
Kids want attention and are sometimes willing to do pretty nasty things to get it. Now slapping IS attention (not very desirable one but...). IMO when a kid wants something doesn't get it and starts whining you have to make it clear in firm speech that there is NO chance to obtain whatever he/she wanted and NEVER give in. Kids ARE smart and if a certain algorithm works one time they are gonna try it again. But if they discover that whining leads to nothing but: "Go to your Room!" they won't see much use in continuing it.
It's also quite smart to explain to a whining kid why whining worked when he/she was a baby (only possibility to communicate and draw attention) but now doesn't anymore (You can talk now. Use that!).