SANTA CLARA, Calif.—The parent company of an amusement park where a 4-year-old boy drowned last week said it is requiring children under 48 inches tall to wear life jackets in all company-owned pools.

The new rule implemented Monday applies to pools at the 18 parks owned by Sandusky, Ohio-based Cedar Fair, including Great America, where Carlos Flores died Thursday.

"We see this as an opportunity to enhance the policies and procedures we already have in place," said company spokeswoman Stacy Frole. "Safety is our priority. I can't stress that enough."

Great America spokesman Gene Fruge said its Great Barrier Reef wave pool remains closed indefinitely since the death, and the life jacket rule has been implemented at the park's Castaway Creek "lazy river" ride.

The rule already was in place in Cedar Fair's Ohio parks, per state law requiring all children under 48 inches tall to wear a life vest or pass a swimming test.

Carlos' mother, Yolanda Flores, 27, said the boy and his 8-year-old sister had been in the wave pool for about two hours Thursday afternoon. When Carlos returned to the pool after a break, his mother lost sight of him for several minutes. The boy's sister found him unconscious.

Flores has criticized Great America for having only four lifeguards on duty for the 350,000-gallon pool. The park has maintained that six lifeguards were on duty.

On Monday, the Santa Clara County coroner's office released preliminary autopsy results that showed drowning was likely the cause of his death. There was no evidence that he was either beaten or held under water, said Lt. James Keith of the coroner's office.