The deficit-reduction package approved by the Senate Budget Committee includes cuts in future benefits for federal Civil Service pensioners, an estimated cut of $700 million in child-nutrition programs and more than $1 billion in unspecified cuts in other income-security programs.

The Senate committee acted with such haste Wednesday and Thursday that many details of the Republican-backed package were not available to committee members at the time of the vote. Republican staff members spent much of yesterday trying to put the final figures together program-by-program.

Democrats on the Senate committee staff and on the House Budget Committee staff were trying at the same time to find out what those figures were.

While the staffs continued their work, Senate Democrats began to take aim at the $966.1 billion GOP deficit-reduction budget, especially the proposed freeze in the Social Security cost-of-living adjustment (COLA) for fiscal 1986.

Senate Minority Leader Robert C. Byrd (D-W.Va.) predicted that the Social Security COLA freeze would "have a difficult time" on the Senate floor. Sen. Donald W. Riegle Jr. (D-Mich.) said he would make an "all-out fight" against the Social Security changes, after having lost a similar fight in the Budget Committee.

Byrd also said he opposed cuts "that might affect the recipients of Medicare," but indicated that he was less concerned about freezing payments to doctors or hospitals.

Under the Senate committee plan, federal Civil Service pensions would be frozen in fiscal 1986. After that, several other changes would take effect, including a cap on cost-of-living increases (2 percentage points below increases in the Consumer Price Index).

The plan, proposed by President Reagan in his budget, also would reduce benefits by 5 percent for each year before age 65 a person retires. In addition, federal workers' contributions to the pension program would grow from the current 7 percent to 11 percent in fiscal 1988.

The estimated cuts in child nutrition are based on figures compiled by the House Budget Committee staff, which spent Thursday and yesterday in consultation with their counterparts on the Senate committee. Their report was forwarded to House Budget Committee Chairman William H. Gray III (D-Pa.).

The impact of the estimated $700 million cut in nutrition programs would include elimination from the program of 2.2 million children, generally from the upper end of the income eligibility scale, according to a study by the Congressional Research Service (CRS).

The CRS also projected that 24 million children would pay more for school lunches and that almost 18,000 schools would be put "at risk," and might be forced to drop out of the program, depriving roughly 5 million children of school lunches.

"I don't think the House will buy that one," Gray said, after asking his staff to double-check the $700 million figure because he did not believe it.

Reagan, in his budget, had asked for the elimination of subsidies for "non-needy" children receiving school lunches and for reductions in both school lunch and breakfast programs.

There also was confusion about projected cuts in the federal Job Corps program. Earlier estimates showed a probable cut of 20 percent in the program, but Democratic staff members on the Senate committee said yesterday that the cut was likely to be closer to 50 percent or more.