It's so hard to tell what course a child's life will take at age 9. Personally, I believe that if you have a son and feel he is going to be a decent athlete in a sport like baseball, football, basketball, etc, then you need to hold off on actual lifting as much as possible. Growth is very important from 9-17 or a similar age range. I believe you should focus on calistenics and more natural strength building exercises, such as the ones you would experience in your sport. For baseball, just hit balls all day, you'll get really strong just from doing that. For football, push bags or tires around, etc. There are lots of things you can have kids do besides weight training at that age. When they reach high school (15 yrs old or so) then they should be ready to start training with real weights.
Now if they have no interest in sports and have more interest in becoming a powerlifter, bodybuilder, non-agility sport, then I'm not the one to comment on that because I have no experience. But I would think starting to lift earlier would not be quite as big of a deal or maybe even necessary.