Assuming you're with your kid on the field, holding his hand, there is nothing you could have done to prevent the kid or you from getting hit. People get hit all the time on the sidelines, he just happened to be in the wrong place at the wrong time. It's no one's fault, it was a freak accident.
I have four kids of my own. I don't pretend to think for one minute that I can prevent them from ever getting hurt or in a situation where I can't protect them. They get hurt whether I'm there or not. That's the nature of it. You just hope it's never serious. The good news is he's OK and that's what we should be thankful for. Seriously, trying to assign "blame" to someone for it because it was a random happening is just ridiculous.