If you take this clip which I got from hardball times and go through it frame by frame (download the gif and then open it with quicktime player) you'll see that Cain starts rotating sooner than Harden relative to foot contact, but because Harden doesn't counter rotate they both end up in about the same position.
Harden really doesn't rotate his upper body at all into foot contact while Cain rotates into foot contact but because Cain has counter rotated neither one is opening up early. Both of them have their arms in similar positions at foot contact but because his rotation starts earlier Cain has to have his forearm pointing down about 3 frames prior to foot contact while Harden's arm is roughly in line with his shoulders 3 frames prior to foot contact. Cain makes foot contact 1 frame after Harden in this clip in order to have them release at almost the same time.
Two different ways to achieve the same goal of staying closed. Harden's is probably a bit more repeatable although I'm not overly fond of what he does with his glove.
One of the interesting things in this clip is the difference in tempo which many people measure from the top of the knee lift to foot contact. They are very different for the two pitchers but it really doesn't seem to have an effect. The exception to this rule might be Roy Oswalt because his tempo is so fast that it makes up for his arm being a bit early at foot contact.
On a good day my son's arm will be in the position at foot contact that Kopp's arm is at 3 frames after first full foot contact. On a bad day that happens even earlier and we need to find out a way for him to have it where Kopp and Cain and Harden have it at first full foot contact. Right now he's just trying to get rid of coming down toe first and trying to stay closed a bit longer to achieve that result.
That brings up another issue. Analyzing video can be very informative but it usually takes a good pitching coach to work with the player to help them make the change. There are very few kids who you can just tell "hey, you're opening up early" and then expect them to be able to make the change. The other thing is that a drill or tip that might work for Harden might not work for Cain or vice versa because of the differences in the way they get to the right position at foot contact and at release.