"The kids were looking at lollies and the father was being served when something flew past and the [4-year-old] boy started crying," the witness said.
"The father went outside and started swearing at a person in a car, which then drove off."
The father and boys went to a house opposite the shopping centre, where it is understood the boys were staying with their grandmother.
The witness said a noisy argument later outside the house had stopped when police arrived.
The Northern Advocate photographed police at the house.
The boy believed to have been hit by airgun pellets was with other youngsters outside the house and seemed to have got over the worst of his injury.
The grandmother and a man - not the father - at the house declined to comment, other than the man saying publication of the incident would start a gang war.
Airgun incidents have had a high profile in recent years.
An undercover policeman was shot dead with an airgun while trying to plant a tracking device on a car in 2008, and an Auckland man was killed with the same sort of weapon in 2010.
Soon after the second death, Police Minister Judith Collins said a move to require people to be licensed to own high-powered air rifles could become law within a few months and would apply to pre-charged pneumatic air guns, but not to older-style, spring-loaded airguns, guns powered by CO2 cartridges, BB guns or paintball guns.