The ban is temporary.
The new rule at the Van Nuys hospital applies to children under the age of 16. Not only can they not visit in-care patients, but they can't be left alone in common areas, like the lobby or the cafeteria.
At Cedars-Sinai, visitors must be at least 18 instead of just 12.
The CDC said more than half of swine flu-related hospitalizations are for people under the age of 25.
Health officials said many people who are 55 and over have some degree of immunity to the virus, perhaps from exposure to a similar strain or vaccine decades ago.
Officials said H1N1 is especially dangerous to children, and doctors are expecting flu-case numbers to double this season.
"Normally about 20 percent of children catch influenza in a typical year. The numbers are expected to be about 40 to 50 percent this year," said Dr. Christopher Tolcher from the American Academy of Pediatrics.
Health professionals are urging the public to get vaccinated, but the problem is the supply.
The U.S. government has ordered 250 million cases of vaccine, but only 12 million have been delivered, as production is taking longer than expected.
L.A. County health officials said they will be opening clinics in Encino and Culver City for uninsured at-risk patients on Friday. They will be opening 13 additional locations by Saturday.