Assigning specific cues to try and make posture and gate changes while athletes are still learning how to run and growing is a good way to artificially set limitations or introduce problems further down the road. This is one of those situations where it’s probably more efficient to aim for those big improvements via general fitness, flexibility and repetition of the act of sprinting versus trying to correct the minutiae which when the runners involved are this young, really doesn’t amount to improvements in time.