According to investigators, the handgun was loaded when the children found it in the bedroom.
The 6-year-old accidentally shot and killed his 5-year-old relative while playing with it.
Daron Mayes was shot in the back of the head around 8:20 p.m. Sunday.
"In situations where the child who is responsible - quote-unquote - is also that young, just understanding the trauma that that child is going through and is going to continue to go through, potentially for the rest of his or her life, it's really a horrible, horrible thing," Chief Assistant Passaic County Prosecutor John Latoracca said.
The owner of the gun, Jalik Jones, was not home at the time. Jones was later arrested and charged with reckless manslaughter.
The prosecutor declined to name the 6-year-old who fired the gun.
"The responsible adults here certainly had no reason to believe there was a loaded handgun in that bedroom," Latoracca said. "They are just devastated, as you can imagine."