As many of you have indicated, all kids are different but here is my story:
Daughter, 21, started out skiing at 7, swithched to the board at 9, extremely smooth and confident and had a very steep learning curve on the board. Won't go back to skiing.
Son, 19, started on a board at 6, struggled for three years before he was able to excell. Now he is wicked. He ended up that way only because he is so G. damned stubborn.
Son, 14, started on skis at age 4, switched to board at 10, was very good and took to it easily and effortlessly, now back on skis. He is very good on both.
When they are that young you have to keep them interested. you can be a sh*tty skier and still have fun. If they are constantly flayling around on an uncontrolable board, always muggin' out they will quit. It is no fun crying on the slope all day.They are easier to teach when they are on skis. Once they understand how the edges work when they pick up the board the learning curve is much steeper. Almost effortless. For the two that did it that way it was a matter of one day for the transition to take hold.
As soon as the kids could make it down the runs without me holding them up, I could slip on the board and still teach them. Even with a leash, mind you I was in soft boots.
I agree with oldacura. Boarding is easier on the knees. I was a skier for 20 yrs before I took up the board.