I think that Pierre has it right--it's the boots. From the POV of Ideokinesis, experts tell us that young kids move better than adults. It is not just youthful energy. They have better posture, which makes movement easier. Young kids are closer to the alignment we have evolved into over millenia. This alignment and posture is something that adults, first as older kids, become "trained" or "habituated" out of in umpteen different ways. To make my point more quickly understood, good posture is something that people in the 3rd World don't lose because they stand, walk and go barefoot most of the time. Put shoes on people, give them cars and let them sit in front of a screen--and they lose their natural posture.
As Pierre explains: Put improperly designed and fitted ski boots on kids and you have a problem. Johann Leitner, the Austrain boot fitter and Nicole Hosp's coach, has worked with Fischer to try and correct what he sees as deficiencies in ski boot design. He explains some of his ideas here: http://www.rexxam.com/e_aone.html.
Another issue is using the wedge in the first place. It tends to naturally put folks in the back seat. I know that Harald Harb and Bud Heishman have worked with a direct to parallel way to ski. Bud explained to me that you need the right hill for this--gentle and wide and long, and for beginner's only. Most resorts have never thought of this necessity.
If you can get kids in the correct boots and not have them wedge, which tends to teach people to sit back, then you can start them off the correct way.
I also think that dchan has a point that kids would be in alignment if we replaced the boots with sneakers. When he makes the observation that some "kids don't have the ankle strength to pull their bodies over their feet" he only underscores Pierre's, or Harb's or Leitner's points about boot design. I believe that you don't need to use any strength to be in proper alignment in a pair of ski boots if they are properly designed and fitted. As I mentioned in some earlier posts, a dancer friend and movement specialist noted that the proper ski positions come from an aligned stance that we have evolved into over thousands of years. Start without that and you've got major problems in your ability to ski.
The industry seems to be changing it's approach to boot design. And some teachers are very progressive and creative in getting to the root of the probelm as ski movement
. But, as creatures of habit, both the industry and the sport are slow to change.