Hi there, this post made me laugh as it brought up memories of a ski lesson I taught with 5-year olds many years ago. FYI, I'm a ski instructor at Winter Park (eight seasons). I often tell this story to my adult lessons. The little boy, Harry was having trouble with mastering his wedge turns, and I could see if was because was "in the back seat". We worked on stance and he said to me "So I shouldn't try to ski like I'm sitting on the potty?"
I'm with the others in that you do need to make sure you have a proper boot fit -- there should be no gaps on the sides, in the heel, at the cuff, or on top of your foot. They should be snug, but not pinching. You may need to adjust your buckles during the course of the day even with the right fitting boot.
But to get back to your original post, proper stance is using your ankles to flex, and put pressure on the front of your boot. The pressure on the front of your boot is what allows you to pressure the front of your ski, thus making controlled turns.
There should only be a slight knee bed, and your hips need to be forward, with a slight tilt of your spine so that you are "nose over the toes."
With the right stance, you should not be putting so much torque on your thighs or knees, which is dangerous because it can cause injury.