Boyfriend puts loaded gun under seat when he gets out of car. 3-year-old finds gun, tragedy follows

A Tacoma, Washington woman and her boyfriend stopped for gas late Tuesday night. While she went inside the convenience store he got out to pump gas. Before getting out of the car he put his loaded, unlocked handgun under the seat of the car. Also inside the car were the woman's two children, ages 3 and 1.
While alone in the car, the 3-year-old boy got out of his car seat, took the gun from under the seat and unintentionally shot himself in the head. The toddler was declared dead at the hospital.
The boyfriend has a concealed weapons permit but reports did not indicate what he feared, why he felt the need to carry a loaded, hidden gun in public, or why he would place an unlocked, loaded gun under a seat and then get out of the car.
Police are investigating but are unclear if charges will be filed. Washington does not have a child access prevention law that would hold an adult responsible if a child gains access to a firearm.
The shooting comes just days after a similar tragedy where a Marysville, Washington boy found an unlocked, loaded gun in the glove compartment of a van and unintentionally shot and killed his 7-year-old sister.