Try not to over-react when something like that happens. Say, oops, gotta watch that boy, in a matter of fact tone. I know it's hard, but they are tough, and if it is just an accident they will be over way before we will. If we start crying or get all nervous, then we can cause the whole situation to be that much worse.
Today, I banged a bucket right on one of their heads, made a good noise too. I just said, "hey, ya gotta watch." Dog ran around and was perfectly fine -- didn't phase her. And even if I suspect an injury, my first response is usually matter of fact, and ignore it. Then, after a minute or more (depending on how bad it seemed to be), I will call the dog over and look at it. Because, especially when there is an injury, the best thing for the dog is to be calm. So the dog needs us to be calm for him.
This is actually good for small children too. Try not to panic. Try not to panic. I had Babsy at the park with the little girls, and they were on the big climby thing, and I had just finished brushing Babsy out, and was walking toward the climby thing, when one of the girls fell off the top of the slide portion. She fell like a ton of bricks onto her stomach into the wood chips.
Try not to panic, try not to panic! Be calm for the girls!