Seven-year-old Jimmy Landry was watching the evening news on television when he invented the bullet "Grabber."

J.P. Cooney dreamed up the "No-Hands Umbrella" because he wanted to play baseball in the rain.

The two Arlington County pupils won first place awards last night in an "Invention Convention" contest that the U.S. Patent and Trademark Office is encouraging schools across the country to adopt.

"We are slipping as far as innovation is concerned, particularly to the Japanese," said Donald J. Quigg, U.S. commissioner of patents and trademarks. "Our kids should spend more time doing what we called in the days gone by 'good old Yankee ingenuity.' "

Last year, 43 percent of the 75,000 U.S. patents granted were issued to foreigners, with the Japanese and West Germans in the lead, statistics show. In 1964, only 20 percent of the patents went to residents of other countries.

In Japan, said Quigg, everyone from kindergarteners to college professors has been involved in invention contests since 1941. The Arlington convention, held for three days at St. Agnes Catholic School, was the first in the Washington area, according to the patent office.

Jimmy Landry, a first grader, said he invented the bullet "Grabber" because of "all the people who got badly hurt on television. If men heard things like World War III was about to break out, they would put it up in the air and it would grab all the bombs and guns."

Standing beside his sketch of an airplane, Landry pointed to the magnetic arms extending from the belly of the craft that sucked up munitions. Promising to tell President Reagan of his invention, Landry noted that "it wouldn't cost much because you'd only need one."

But not all 200 inventions spread across the schools' gymnasium and library tables yesterday were born of such lofty ideas. J.P. Cooney, 9, just got tired of holding his umbrella, so he strapped it to his back. Other students concocted models of trucklike litter eaters, fingernail polishing machines and bicycle helmets with lights.

"Even by the eighth grade, the kids get more practical," said Sister Mary Anne Glaser, the school's principal who organized the convention after attending a national conference in Crystal City last October promoting student inventions. "The whole idea is to let them be uninhibited and creative," she said.

Distinguished from science fairs, where students often build intricate devices that prove scientific theories, invention conventions stress ideas that address problems. They do not require actual working devices, but rather depend on models or sketches.

"Science fairs tend to be elitist because the father who is a mechanic ends up building his kids' car model," said Marion Vosberg Canevo, principal of School 61 in Buffalo, who is credited by Quigg with spreading the invention fairs around the country.

Three years ago, Canevo started an invention contest in one Buffalo school, and since then she has been made coordinator of creativity conventions in 85 Buffalo schools and others in western New York.

"New Jersey has a statewide program," Quigg said, "and they are doing it in Ohio now." But American children still have a lot of catching up to do, he said.

As an added incentive at St. Agnes, the first place winners announced at a parent-teacher meeting last night received a $25 bank account or a $50 savings bond. The entries were judged by Washington area businessmen. Regional winners in New York have won as much as $500 and a trip to the National Inventors' Hall of Fame in Crystal City.

Among the winning inventions in Buffalo: Mittens with a sewn-in Kleenex dispenser for the child with a runny nose; a tall-hanger-for-short-people for the child who never hangs up his clothes because he can't reach the closet rod, and the floating puzzle for those who like to solve jigsaw puzzles in the bathtub.