I have a 7 year old and 4 year old daughters. They both started at about age 3 on the boom and progressed from there. My 7 year old started droppping a ski when she was 5 and learned to get up on 1 at 6. By the end of this summer she was knocking down the mini course pretty easily and starting in on the real course at the end of the summer. I agree with fu_man. Encourage the heck out of them but don't push them if they don't want to go. My 4 year old was all gung ho early in the summer so I pushed her a little hard and then she didn't want to go very often for the rest of the year.
I started my 7 year old on two skis in the mini course about as soon as she could get outside the wake. I let her just poke around at the buoys and if she didn't feel like going after them, so be it. When she was learning to slalom, I let her pick and if she wanted to ski on two she would. If she wanted to slalom, she would. She was still doing that at the beginning of this year from time to time. I think it helped her to be really solid on one when she finally gave up the other ski for good.
It also helped to show her some videos of really amazing kid skiers like Drew Ross' daughter so she could see what elite kid skiers look like.
I think the biggest thing is to always have it be fun, encourge them when they do something well, and push them to try something new when they show they want to do it. I guess the only other thing that worked for me is to not make things too intimidating or seem too complicated. We've gotten so many little kids up on one ski lately when they just go into it thinking it isn't that hard to do.