An acceptable, crash-tested car seat or restraining harness is vital for the safety of any pre-school child you have as a passenger in your car.

Without special restraints, small children are exposed to the possibility of being smashed against a bunch of unforgiving knobs and protrusions on the dashboard.

According to the National Highway Traffic Safety Commission, car crashes are "the leading cause of death for children ages 14 and under." Some 4,100 children were killed in car crashes last year and hundreds of thousands were seriously injured. If you don't have a specially designed safety seat or harness, says Dr. William Haddon Jr., president of the Insurance Institute for Highway Safety, use your car's regular restraint belts with cushions or a bex to serve as a booster seat.

Dr. Haddon says you should, whenever possible, put children in the back seat where they're much safer than up front. He warns, "Keep children out of the backs of vans, pickup trucks and station wagons."

"No child," says Dr. Haddon, "should be left unrestrained in a motor vehicle under any circumstances."

It's up to parents to make sure thierchildren are securely restrained when they're traveling in a car. If small children are being driven by others to preschool classes, they should be properly restrained. "Check on it. "One of the problems," says Debbie Richards, executive direction of Action for Child Transportation Safety, "is the fact that not all seats and harnesses fit all cars or all children."

When you're out buying an infant seat, a safety seat or safety harness for your child, Richards says you should ask the salesperson if it's possible to try it out in your car, on your child, before you buy.

Some seats don't work well with certain lap and shoulder straps in some cars. And some restraints don't fit some children -- especially bigger children.

The seat should be installed exactly according to the instructions. If it won't work, you should try another model.

Also, some seats and harnesses are much better made -- much safer -- than others. You can get a list of the best, tried and tested seats and harnesses from: Physicians for Auto Safety, P.O. Box 208, Rye, N.Y. 10580. Send 35 cents and a self-addressed, stamped, business envelope.

One problem may be solved early this summer when a manufacturer plans to market a special "booster" seat for larger children from ages 4 through 6. These children are usually too big for the safety seats and harnesses and have to use regular car lap-and-shoulder restraints.

The children often aren't big enough for these car restrains to fit properly. You can help things along by using firm cushions or a little box for the child to sit on. But this makeshift arrangement is not entirely satisfactory.

"The new booster seat seems to be the answer for larger children," says Richards. It will be manufactured and marketed by Century Products, a company that already makes safety restraints for smaller children.

Meanwhile, the government is coming out with a new rule that will require child safety seats and harnesses to be able to withstand crash impacts at speeds up to 30 miles per hour. The new rule, which will go into effect next June, includes car beds, infant carriers, seats and harnesses used in motore vehicles for the protection of children up to 50 pounds.