WASHINGTON (AP) _ President Reagan on Saturday exercised a pocket veto of legislation that would have limited advertising during children's TV shows and required stations to provide informative programming for youngsters.

The president said in a statement that while he supports efforts to improve the quality of children's programs, ''this bill simply cannot be reconciled with the freedom of expression secured by our Constitution.''

The bill, which passed the Senate on a voice vote in October and the House by a 328-78 margin in June, would have reimposed advertising limits abandoned by the Federal Communications Commission four years ago. The FCC had said the marketplace itself could best regulate commercials.

''While I applaud efforts to increase the amount and quality of children's television programming, the Constitution simply does not empower the federal government to oversee the programming decisions of broadcasters in the manner prescribed by this bill,'' Reagan said in his statement.

Children's TV advocates had pressed for passage of the legislation, maintaining that under the FCC's philosophy of deregulation, advertising during children's shows has increased and the quality of the programs has deteriorated.

The bill would have limited commercials in children's program to 10 1/2 minutes per hour on weekends and 12 minutes per hour on weekdays. Stations would have had until 1990 to comply with the advertising limits.

It also would have required stations to ''serve the educational and information needs of children'' in their overall programming. The FCC would evaluate those efforts in considering whether to renew stations licenses.

Reagan, in his veto message, said that ''conditioning license renewals upon the federal government's determination as to the adequacy of a licensee's programming would violate the First Amendment.''

''It would inhibit broadcasters from offering innovative programs that do not fit neatly into regulatory categories and discourage the creation of programs that might not satisfy the tastes of agency officials responsible for considering license renewals,'' he said.

FCC Chairman Dennis Patrick had said the legislation was ''unnecessary and ill-advised,'' particularly because the commission was considering taking action on its own related to the commercialization of children's television.

The legislation was a bipartisan compromise that had been stripped of a proposal to prohibit programs that feature toy manufacturers' products. Critics call such shows ''program-length commercials.''

Edward O. Fritts, president of the National Association of Broadcasters, which helped craft the compromise, said of the bill: ''While we recognize this legislation charted some new territory we were willing to accept its outcome.''

''Frankly, we expected the president to sign it.'' Fritts said. ''Under any circumstances, we are very sensitive to our responsibility to provide quality programming to children and we will continue to do so with or without legislation.''

Reagan also signed a number of other measures that won legislative approval in the waning days of the 100th Congress:

- A law banning the sale of lawn darts, a game that is blamed for the deaths of three children and injuries to thousands of others. The bill requires the Consumer Product Safety Commission to carry out the ban, which is moving to do so on its own.

- The so-called ''Bork bill,'' designed to protect the privacy of video store customers by prohibiting the unauthorized release of information about what tapes they rent or purchase.

The bill grew out of last year's hearings on the nomination of Robert H. Bork to the Supreme Court. During the deliberations, a weekly newspaper in Washington, D.C., obtained and published the video rental records of Bork, whose nomination ultimately was rejected by the Senate.

- A law banning the shipment of real-looking toy guns and of imitation guns unless they have a blaze orange plug in the barrel. The law was enacted in part because of shootings in which imitations were mistaken for real weapons.