Senior Sergeant Tony Bates said police were investigating, particularly how the air rifle ended up in the hands of the children.
Senior Sergeant Luke Shadbolt said people had a tendency to treat air rifles like toys but they could cause injury.
"The rules that apply to any other firearm should be applied and air rifles should certainly be kept out of reach of children."
Mr Shadbolt said it "is not uncommon" for people to be injured through the irresponsible use of an air rifle.
A person over 18 did not need a firearms licence to buy an air rifle, while a person under 18 did require a licence, he said. Firing an air gun in a way that might endanger or frighten anyone, or harm property, could mean a fine of $3000 and/or imprisonment for up to three months.