Malnutrition found in some low-income children recently by Boston-area pediatricians is a sign of the potential harm of Reagan administration budget cuts in the women, infants and children (WIC) food program, a nutrition advocacy group contends.

But President Reagan, returning to the White House from Camp David yesterday, disputed any suggestion that his budget cuts may be responsible for any rise in child malnutrition.

"I think it's a great exaggeration. We have not reduced spending to below what it was, we have only reduced the rate of increase in spending. We are spending more by 4 1/2 percent next year than we did this year," Reagan said in response to reporters' questions about the report, released yesterday by the Food and Research Action Center.

The Washington-based group cited the Boston cases as evidence that Congress shouldn't accept the proposal to cut WIC from $900 million to $600 million in a health block grant.

The program was cut only about 4 percent last year. WIC, which supplies supplemental nutrients to poor pregnant and nursing mothers and their young children, was begun in 1972 and now serves about 2 million people.

Dr. William Bithoney, director of the child health program at Boston's Children's Hospital, said in a telephone interview over the weekend that 10 of the 100 youngsters he examined last week were suffering from mild malnutrition.

He said two youngsters who were dropped from Boston WIC programs because their parents failed to comply with exam or inoculation schedules suffered sharp weight losses and infections. They were reinstated to the program.

Bithoney acknowledged that these cases of malnutrition weren't caused by budget cuts. But he expressed fears that the cases "may well be symptomatic of what we might see across the country" if the cuts go through.

In a statement, the nutrition group also noted three cases found by Dr. Deborah Frank and by Boston City Hospital's "failure to thrive" teams, which deal with children who don't grow at their age group's expected levels.

Frank cited children who dropped from the WIC program, then showed loss of weight and illness. "We have an extremely fragile system of food assistance in place now," she said in the statement. "The problems of malnutrition and 'failure to thrive' that we are seeing today will only be made worse by budget cuts . . . ."

Jerianne Heimendinger, director of nutrition service for Rhode Island's health department, told the House Education and Labor Committee last week that WIC is a success and shouldn't be cut. The committee has recommended that the House Budget Committee reject the proposed Reagan cuts in WIC and other nutrition programs.