Police said the boy and his 1-year-old sister were in the car with his mother while his father was inside a nearby business.
The father had placed his lawfully owned and carried handgun under the seat before getting out of the car, according to Police spokesperson Casey Liening.
The boy got out of his car seat and grabbed the gun, which “accidentally” discharged, striking the mom, Liening said.
The mother is expected to recover.
The police said it was “too early” to charge anyone.