Have you ever accidentally hurt a child?

I don't have kids of my own yet but I work with 2 year olds at a daycare. We were getting them settled for nap time and this one little boy was running around. I take him by the arm to sit him down and he unexpectedly turns and smacks his head on the rug...I can still hear the sound. He cried so I held him and comforted him until he stopped, I looked at his head and it was only a brush-burn, no bump, no bruse. But I felt sick...I still do and this happened yesterday. I talked to the other workers, my boss...anyone who would listen and they all say its ok and it happens to everyone. How do you get over the guilt?