The federal government, in major new changes for the nation's school-lunch program, wants to call ketchup and pickle relish vegetables, offer tofu as a substitute for meat and serve peanut butter or nuts as main dishes at noon.

These and other such redefinitions of federal child nutrition standards have been proposed by the Department of Agriculture as part of the administration's budget-cutting program.

The changes, proposed formally last week, would affect more than 27 million children, many of whom now get free or reduced-price meals, in school-lunch and breakfast programs and in the food program in child-care centers.

If adopted as proposed by the department, the changes would mean that students will get less food than last year, in addition to having to pay more as a result of reduced federal lunch subsidies authorized by Congress.

The department's proposals have drawn strong protests from nutrition and agricultural commodity groups, some of which met yesterday to draw up plans for battling the administration's new plans for the feeding programs.

"There is no question that the health and nutritional status of children is being compromised as a result of these new proposals," said Nancy Amidei of the Food Research and Action Center (FRAC). "We do not believe the department has looked sufficiently at other possible cost savings as the Congress ordered."

Marshall Matz, an attorney for the national school food services association, added, "We are extremely concerned about the nutritional integrity of the school lunch program. The prices of lunches are going up--we think an average of 25 to 30 cents nationally--and now the department is proposing a reduction in the amount of food that can be served."

Congress, in reducing child-nutrition spending from about $4.5 billion to $3 billion for fiscal 1982, also directed the Agriculture Department to find other ways to economize, but stressed that the department should maintain the nutritional value of school lunches. The department was given three months to comply.

The result, according to Mary C. Jarratt, assistant agriculture secretary for food and consumer services, is a series of approaches to lessening reporting and record-keeping at the local level, plus smaller portions of food and the use of substitutes for more costly items.

Under this plan, tofu, a soybean curd, would be permitted as a meat substitute; yogurt could be served instead of meat or milk; cottage cheese could be offered instead of lean meat; peanuts, either whole or in butter form, plus other nuts and seeds, could be used instead of meat.

In a departure from its traditional two-vegetable requirement for the standard lunch, schools could credit such items as ketchup or pickle relish as one of the vegetables. An order of french fries, for example, smeared with ketchup, would fulfill the two-vegetable fiat.

By FRAC's calculations, the reduced-sized lunch for secondary school students would contain the equivalent of one-fourth of the meat in a McDonald's "quarter pounder," served on half of a roll, with six french fries, nine grapes and part of a glass of milk.

Another meal devised by FRAC nutritionists, using the proposed new standard, might contain three-fourths of an egg on one slice of bread, some carrot sticks and a wedge of apple, with a partial glass of milk.

"We feel this is compromising children's health," Amidei said. "The proposed new standards will not provide one-third of the daily nutritional requirements of children. On top of this, in many instances, some poor children receive up to half of their daily food from the school-lunch program."

The department's formal publication of the proposals indicated that extensive changes in reporting and record-keeping rules would provide some savings to school districts, but not enough to satisfy budget requirements. The reduced-size meals were the only remaining alternative, the statement said.

USDA's proposals quickly stirred concern on Capitol Hill. Rep. Carl D. Perkins (D-Ky.) has scheduled three days of education subcommittee hearings for later this month to delve into the revisions of the nutrition program.