According to the details on the video below, this event happened in Eugene, Oregon, on August 4, 2013. The police helping the Department of Human Services to remove the ten year-old child from his mother's custody to be put into the custody of her former boyfriend.
(Note- Correction on the age of the child changed in headline and article due to conflicting reports, it is now believed the child was 10)
The child is being pulled along by the cop, yelling for help and saying "save me." When witnesses yell at the cop about hitting a child he defends his actions because the child bit him.
In the video the child bites the police officer and the cop backhands the child.
At the 39 second mark the person with the camera stops running and the camera straightens out to film the whole event.
(Strong Language Warning)