Automobile accidents are the leading cause of death in children under 14 -- and now a new study shows that the children at greatest risk are those under three months old.

Yet only 7 percent of American children wear seat belts or ride in protective cars seats.

"Our priorities are hugely and tragically out of whack," Dr. William Haddon, of the Insurance Institute for Highway Safety, told the D.C. chapter of the Amercian Academy of Pediatrics recently.

In 1977 -- the most recent year for which statistics by age are available -- 4,614 children 14 or younger died in motor vehicle accidents, according to the National Center for Health Statistics. Although adults have a higher death rate from car accidents, the chances of a child being killed in a car accident -- 9 deaths per 100,000 children -- were almost twice that of dying of cancer, the number two cause of death in that age group.

In very young children, the risk was even greater. The death rate for babies between one and three months of age is 12 per 100,000, according to an analysis of the 1977 statistics by Susan P. Baker, an associate professor at the Johns Hopkins School of Public Health.

Baker and others believe that the major reason for the high death rate is that infants are ususally held on a paent's lap while riding. During a crash, a baby'body becomes "a flying missile," and the adults's body being propelled against it "guarantees that the child will be crushed," said Joan Claybrook, administrator of the National Highway Traffic Safety Administration (NHTSA).

Crashes filmed by the NHTSA, using child-sized mannequins riding in real cars, provide horrifying proof of what happens to an unrestrained child during an accident at 30 miles per hour. In one film shown pediatricians, a toddler-sized mannequin sails from the front seat into the windshield, shattering the glass with its forehead and snapping its neck far enough back to break the spine.

In another, two mannequins the size of a three-year-old and a six-year-old shoot back and forth over the floor of a van during impact bouncing off sidewalls and seatbacks.

In a third film, two mannequins riding in the back of a station wagon fly out the rear window during the crash, with the three-year-old-sized one landing on its head in the road.

A study by Baker of all auto accident deaths of Maryland children over a five-year period showed that 80 percent had severe head injuries, including skull fractures and lacerations of the brain.

Claybrook said the proper use of a seat belt or restraining car seat could have eliminated or reduced more than half the head injuries.

There are a number of car seats for babies and toddlers on the market, and the NHTSA tests their safety and attempts to promote their use through pediatricians, hospital obstetrics departments and other groups. Studies have shown the seats protect children better than seat belts, which are designed for adults and do not always restrain children adequately, even though the belts are much better than using no restraint at all.

Yet educational campaigns to increase the number of drivers restraining their children have had little success, according to Allan Williams, a behavorial scientist at the Insurance Institute. Laws work somewhat better: bIn Tennessee, after passage of a 1977 law requiring that children under four be restrained while riding, the proportion of children riding restrained rose from 9 percent to 22 percent, Claybrook said.

Automobile manufacturers have not cooperated so far with safety campaigns. A NHTSA study of 25 1980-model cars found that 16 had seat belts too short to accommodate one or more of the most popular car seats. Many cars also lacked locking devices on seat belts, which are necessary to keep the car seat in place during a crash.

Claybrook also criticized manufacturers for designing dashboards with protruding knobs and hard surfaces that could injure a child's face during a collision. She pointed out that most areas of car dashboards ar not regulated by safety standards.

"Manufacturers are not using the human engineering technology and talent that is so readily available," she said.

But statistics show that the greatest reduction in accident deaths of children could be achieved simply by parents strapping them in.

"At the minimum, you should have your child sitting in the back seat with the belt pulled tight," said Williams.