While the federal government has successfully promoted an impressive expansion of on-site child-care centers at federal agencies in the last three years, one top official said yesterday that too many of the centers are not practical for workers because they cost too much.

"Many of the lower-income employees have not been able to take part in the programs," Constance Berry Newman, director of the Office of Personnel Management, told the first national conference of federal child-care providers. She called on them to find ways to make the programs more equitable.

The two-day conference, sponsored by the General Services Administration, which is responsible for providing space for child care in federal facilities, brought together about 300 center directors, care-givers and federal agency coordinators.

"Child care has been one of the most successful programs and initiatives ever undertaken" by his agency, GSA Administrator Richard G. Austin told the conference.

In September 1987, when the GSA became actively involved in promoting on-site child-care centers at agencies, only 12 existed in GSA-controlled space, Austin said. Now there are 66, with 50 more planned by the end of fiscal 1993.

Cost has been a factor that has concerned administrators from the start. When the GSA opened its own child-care center in 1987, for example, its rate for infants was $115 a week, out of range for many employees.

One of the goals of a five-point plan outlined by Austin yesterday for improving child care at federal agencies was to make programs more affordable by providing scholarships to federal employees.

Other points included increasing salaries and benefits for center employees to stem turnover rates, using accreditation criteria of the National Association for the Education of Young Children, ensuring health and safety and sharing ideas through the federal government's national network.

Education Secretary Lauro F. Cavazos, who also addressed the conference, announced four new child-care studies being conducted by his department. One is a profile of child-care settings across the country, while another is a review of before- and after-school programs. Two others will look at preschool programs, one an analysis of what succeeds in preparing a child for school and the other to determine how to help children retain the advantages they get from preschool programs such as Head Start.