Manuel Murillo, 37, is accused of placing his loaded deputy firearm into his 8-year-old relative's backpack before dropping the child off with a caretaker on April 11.
That caretaker noticed the weapon about 25 minutes after Murillo left and contacted Fullerton police, who is investigating the case.
No further details on the incident were released.
Murillo faces a maximum sentence of one year in county jail. A pre-trial hearing is scheduled for Aug. 23.