When the mother of the victim came back into the room and asked what happened, one of the children told her, "Anthony shot him," police said.
The child was taken to Advent Health Lake Mary ER for treatment, records show.
Hospital staff treated the child and found a fragment from a .22-caliber round.
Police later returned to the home and arrested Knuth.
Knuth's sister said it's not an excuse, but her brother's alcoholism has gotten worse since the 30-year-old's live-in girlfriend died in April.
"From what I understand, he showed the kid the gun, then loaded it and then shot him in his calf," said Knuth's sister, Heidi Netti.
One witness said as several children played with Nerf guns, Knuth told the children he had a real gun and was going to put a real bullet in it.
After the shooting, Knuth allegedly told police he was under the influence of multiple shots of whiskey and a 24-pack of beer.
Officers said Knuth admitted to owning several firearms, including a .22-caliber rifle.
“Unfortunately, when my brother gets drunk, he is wacko,” Netti said.