A Gaithersburg mother, speaking at a Capitol Hill news conference, called yesterday for support of legislation that would award federal funds to states that require the use of child restraint seats and said one of the devices saved her infant's life.

Marjorie Zarin and her daughter, 15-month-old Stacy Zarin, were among persons invited to the Rayburn House Office Building by Rep. Michael D. Barnes (D-Md.) in support of his legislation. It would provide $25 million from the federal Highway Trust Fund to help develop well-rounded programs and to augment laws mandating the use of child restraints. The funds could be used for such things as public information, enforcement and safety-seat loan programs.

Most states that have enacted laws "lack the resources for the next big step . . . the implementation of long-term comprehensive programs to support these laws," Barnes said.

Safety-seat loan programs are already in operation in the District, Maryland and Virginia and many of them are run by volunteer organizations.

The programs work differently in each jurisdiction, but most provide rental of the safety seats for $10 to $25 and several provide free loan of seats to low-income families.

In Virginia, for instance, the state has provided about 5,500 seats free-of-charge to low-income families through local volunteer groups, county sheriff's departments and the state Division of Motor Vehicles, according to a spokesman for the Virginia Department of Transportation Safety. Maryland and the District have had similar programs in effect for several years.

Another speaker yesterday, John F. Herrity, chairman of the Fairfax County Board of Supervisors, announced the start of a loan program in his jurisdiction. And a spokesman for the Automotive Trade Association of the National Capital Area said the group would donate 50 car seats to get the program under way.

Other loan programs in the area include the Montgomery County "Kids in a Safety Seat" program, run from Shady Grove Hospital; several Jaycee programs in Prince George's County; Project Safe Child in D.C. and the Arlington Hospital Auxiliary program in Virginia.

Barnes said that in the last five years, nearly 3,400 children under the age of five--83 of them in Maryland and Virginia--were killed in motor vehicle crashes, according to the National Transportation Safety Board. Figures were unavailable for the District.

In an effort to prevent these deaths, 40 states, including Maryland, Virginia, and the District, have passed laws requiring the use of child restraint seats and safety belts for children. The D.C. law goes into effect Friday; the Maryland law Jan. 1, 1984. The Virginia law has been in effect since last January. All three call for a $25 penalty for violation.

Barnes said proper use of child restraints in automobiles is 90 percent effective in preventing deaths and 67 percent effective in preventing injuries.

Zarin, in moving testimony, described the rainy October afternoon when her car, forced over the median strip by a vehicle exiting from I-270, was hit broadside by a car and a Montgomery County school bus. Her daughter was strapped in a child restraint seat in the back of their 1980 Pontiac, she said.

"As my car jumped the median I can remember trying to turn around and see the baby one more time before we died," Zarin recalled.

But after the impact, Zarin crawled into the back seat and found Stacy "still restrained, and most important, still alive." She and Stacy suffered nothing more serious than bruises and abrasions, she said.