Aside from form issues, I spoke with CJ Murphy (strength coach / owner of TPS) a few months ago about what would be acceptable training for a youth and as others have mentioned there is a relationship between strength and speed. The problem is that with youths you don't want to do anything with too high of an intensity just because of the risk of damaging their bodies before they reach puberty.
This leaves you with dragging a light sled or pushing a prowler, bodyweight squats, plyo-jumps...or basically playing on a playground like little kids do.
I would actually be interested in seeing how much the body adapts with light resistance training at the youth level. I mean, these kids don't really have the hormones needed to make great gains, but they are still growing, so you've got to figure that specialized training would yield an increase in performance.
I was looking into all of this at the end of the season last year and I ultimately settled on just relaxing until my boys get a little older...at that point I'll introduce them to weight training.