The young girls were apparently the target of the same man prowling Whittier neighborhood parks for young victims earlier this month.
The older girl, accosted Thursday afternoon, managed to get out of the car and free the 7-year-old and both girls ran in different directions to escape the suspect, Officer Brad White said.
The 11-year-old ran from Founders Park and called police when she got home. The identity and whereabouts of the younger girl are unknown.
"We have not received any information on the 7-year-old," White said, adding there are no reports of missing children in the suburb 20 miles southeast of downtown Los Angeles.
The girls do not know each other and the 11-year-old wasn't hurt.
Extra police were on duty Friday in the hunt for the suspect in a red convertible with a tan top. He was described as a 25- to 35-year-old, Hispanic, about 6 feet tall with a medium to heavy build. School superintendents have also been alerted.
The 11-year-old girl said he had short black hair with three dots tattooed on his left hand.
"It is very unusual for Whittier," White said. "We're taking it very seriously. We are following any leads we have.
The same man approached three teenage girls earlier this month, authorities said.
Just before noon on Feb. 5, a man matching the same description and driving a red convertible pulled up to a 17-year-old girl walking near Broadway Park and asked her if she wanted a ride.
"After the victim told him no, he proceeded to stop the car and open the door," Lt. Kent Miller said in a statement. "The female fled and called the police."
About five hours later, a witness called police and said two teenage girls near Central Park were arguing with a man who had ordered them into his red convertible, White said.
The suspect drove away and the girls left on foot.
White said the witness didn't know the teen girls, and they didn't call police.