"It doesn't have an external safety or anything like that," Lt. Sierra Brucia said. "The gun functioned how it was supposed to. When the trigger was pulled, the gun went off."
The officer was showing the department's SWAT truck and his gear to children at an event called the Reading Roundup Aug. 24 when the incident occurred, KVOR-TV reported.
The child who pulled the trigger was 6-to-8 years old.
The bullet hit the officer's leg and he was taken to a hospital with minor injuries.
The officer, who has been on the SWAT team for five years, was later released and is back on duty.
KVOR-TV said the department is investigating the shooting to see if certain procedures need to be changed,