Kids have great visual imaginations, so make use of it with this drill: imagine there's a coathanger in your back and your arms are being hoisted up by a puppeteer. Run a short 200 metre course imagining you're still suspended on a rail, keeping a straight line right down your form from head to toe. Practice like this until it feels natural, always keeping upright and erect. Keep arms at your hips and don't pump too hard. Accelerate by pushing forward fro the knees down. It's how professional athletes are trained at first. Balance, posture, control are core elements of any training to articulate to kids. Enjoy!