awww, that's hard. I feel bad when kiddies get hurt too--thank god, not too many injuries, but I did have a little boy fall at the playground and he had such a bad bump that I called his mommy and she took him to the hospital (he was fine btw). I felt TERRIBLE!
We just have to do our best and realize that they will get hurt from time to time and that no matter how close we watch, sometimes there is nothing you can do about it. Don't beat yourself up about it--his mama is right, I'm sure he'll have more!