@pearsaab : I took a look at your photos and clicked on a few to get some info on them. Our shooting style is very different. It looks like you were zoomed in to the max of 70mm and you set the lens to f/7.1. I usually shoot somewhere in the 40mm to 50mm range, vertically, with the runner taking up ~70% of the frame. My runners are much closer to me. The lens is at f/6.3, just a little faster than f/7.1.
In my case the only thing that helps me a little bit is setting the focus back manually to the minimum distance between shots (if possible). When AF is activated the focusing mechanism moves toward infinity while the runner is moving toward minimum focus. Both meet in the middle and *click* goes the camera.
@UncleVanya : I don't think prefocusing and waiting for a runner to hit a spot would work in my case. The kids run around the field in a counter clockwise direction. I walk around it clockwise. That lets us see each other. The kids love this because when they see me coming toward them they are ready to jump, wave their arms in the air, smile, etc. In most cases I can't predict what they will do or in which direction they go. It's like photographing bees. Prediction is hard.
One exception for the prediction - girls are more likely to stop and pose with their friends or an adult/teacher/parent whereas boys are more likely to ham it up and do something heroic and action oriented. Girls are more likely to pile into one huge gaggle but boys will pick one to three friends for their group shot.