Police said that at some point, the adult left the children unsupervised, and the 9-year-old girl was shot by a sibling accidentally while the sibling was shooting at a target.
"It is outrageous that this adult provided firearms and ammunition to these young children," said Deputy Chief Todd Hutchinson. "Especially given his past arrest and conviction."
Police said the family transported the child to the hospital.
The child was critically injured and is in stable condition, officers said.
After a lengthy search, officers found several firearms on a trail hidden under a disposed tire in the wooded area, officials said.
No other details were made available.