Officer Roberto Machuca was arrested June 27 and booked into the Orange County Jail, according to Garden Grove police. Investigators were notified of the allegations on June 26.
Jail records indicate he was released the day after his arrest.
Machuca was relieved of his duties as a police officer and placed on administrative leave, police said.
The minor was participating in the department's explorer program, which is for youth interested in law enforcement.
"The safety and security of our youth volunteers remains a top priority," a police spokesperson said in a statement.
Machuca, 32, has been with the department for several years.
The investigation remains ongoing. Garden Grove police said no additional information is being released at this time.
Police said the case will be forwarded to the Orange County District Attorney's Office once the investigation is complete.