Criminal negligence
Anyone who knows anything about automatic weapons knows they climb because of the recoil. An eight-year-old doesn't have the physical strength to counteract this tendency, nor the experience to understand its danger.
Besides that, once a finger is on the trigger of an automatic weapon, fire is fast and continuous; inexperienced people tend to be startled enough to fire until the clip is empty.
An adult's hand should have been on the barrel, holding the weapon down. There is absolutely no excuse for this.
His poor mother.