How about teaching aids?
There is a wide variety of teaching aids out there, from harnesses to contraptions that prevent crossing the skis or prevent the child from "running loose". All of these can have specific positive applications, but can also have downright negative impact on the learning curve when used indiscriminately.
"Edgie Wedgies", for example, are useful when a child has limited strength to maintain a wedge. Using a Edgie Wedgie, the child can focus on feeling the steering movement in a turning wedge, without exhausting himself/herself in maintaining the wedge itself. If left too long in such a device, however, the child will simply learn to lock his / her skis against the device without having really learned the requisite skill.
Harnesses should be used only sparingly, and only in very specific cases. I have observed that, when used indiscriminately, they can promote out of control, straight line skiing, and backseat skiing, as opposed to speed control through turn shape and rhythm. I have used harnesses on occasion, but carefully, keeping the lines loose almost all the time, and using them only to provide reassurance to a worried child, or to give the very initial impulse into a turn. The bottom line is, if you need a harness to heavily control your child's skiing speed and trajectory, he/she is not quite yet ready to be on the terrain on which you are skiing!
I am now playing with a new teaching aid that provides control by attaching to the boots - and leaves it to the child to manges his-her balance.... check it here:
My favorite teaching aid is actually my ski pole providing a direct connection and support to the child, held via a loose attachment to another pole, or simply held by the child alone as a way to provide a balancing element, and to control arm movement .
(see movie clip)