Jamaria Cobb and Armand Bright, 52, were both arrested on suspicion of child abuse causing great bodily injury or death. They remain in custody on $1.3 million bail.
The incident happened on Thursday just before 5:30 p.m. at a home on Victoria Street.
According to the San Bernardino County Sheriff's Department, Cobb and Bright were at home when the child found the gun hidden between two couch cushions and shot himself.
Deputies performed CPR on the boy when they arrived at the scene, but the child later died at a hospital. Investigators said a second child who was inside the home was taken into the custody of Child and Family Services.
Anyone with information about the case is being asked to contact the Specialized Investigations Division at 909-890-4904. Those wishing to remain anonymous can call or text REPORT to We-Tip at 844-909-3006.
Tips also can be submitted online at https://mobile.catapultems.com/sbcsd/Sites.