I struggle with dynamic mode. I have had some success with it, usually when my daughter was off alone on the field or birds in the sky. Other times, dynamic mode works out. I have difficulty with the best application for both of these modes. With scenes like this, you may be stuck picking the initial focus point anyway. I would even give auto area a try. People tend to stay away from auto area, but I think in most cases it would pick the closest kid to focus on and that could work out for kids running at you.
Pay attention to menu a3. Usually best to keep it off. It introduces a delay to continuous focus. It can cause problems when trying to track. Keep it in mind though if you find focus is distracted by other kids in the scene.
Horoscope already mentioned this document:
http://www.pixelfinesse.com/_docs/D7000_AF_Explained.pdf
I re-read it often.