FROM THE FOLKS who brought us ketchup as a vegetable in the school-lunch program comes a really terrific new idea. For the new round of budget cuts sought by the president, the Agriculture Department is considering adopting a cost-saving move that could jeopardize the mental development of tiny tots.

We may be on the verge of Dr. Reagan's "Prescription for Economic Recovery Without Harming the Poor." To save money in the Women, Infants and Children supplemental food program (WIC), the wonderful solution being talked about is to lop poorly nourished children off the rolls after they get to the sturdy old age of 1 year. Brilliant idea. The lopped-off children probably will never get enough nourishment to be smart enough to know what hit them.

The WIC program provides nutritional supplements for poor, undernourished pregnant women and their infants and children with special nutrition problems. Under the program, the poor mother whose infant is undernourished may be given a voucher by a health professional that will entitle her to purchase infant formula and other more nutritious food from a supermarket for the first few years of the child's life.

The program is difficult to qualify for. Not only must participants be poor, they must also be shown to be "high risk" cases -- meaning they are likely to have serious nutritional problems if they don't receive a better diet.

When this program was started in 1972 under Richard Nixon, the White House Conference on Food, Nutrition and Health looked at children who were suffering from an acute form of protein-calorie malnutrition syndrome. Authorities argued that, while many children appeared to recover from malnutrition, they never recovered attained normal growth. Some remained permanently mentally handicapped.

Programs like WIC have succeeded in sizably reducing the likelihood that low-weight babies will suffer retardation.

Before the first round of budget cuts, this Agriculture Department program served 2.2 million people and cost $1 billion a year. When they changed the eligibility rules last week, making it tougher to qualify, one official estimated that thousands of persons probably would be dropped from the program.

Under the rules they are considering now, an 11-month-old baby could be helped while a 13-month-old could be rejected.

John Bode, a deputy assistant secretary in the Agriculture Department, says this option is being considered because the greatest benefit to the infant accrues when it is less than 1 year old. There is less of a risk thereafter, he said. He stressed that the cut is being considered as an "option" and not as a "policy position."

This theory is fine -- when you are dealing with poor persons whose food stamps haven't already been cut off and who have to deal with inflation. It is fine -- when those affected do not have to cope with poor education, too many children, low motivation, and chronic health problems already. It is fine -- when poverty hasn't already driven recipients to sell food stamps just to be able to pay the rent. Even in better economic times, children under 5 years old were covered by the WIC program.

Then there are the many children with special health problems. Take the case of a child with PKU. PKU stands for phenylketonuria, a metabolic disease that can cause retardation and behavior problems. Its effect can be prevented or greatly minimized through the use of certain vitamins and a change in diet. In fact, an l8-month-old with PKU needs the canned formula as much as a 9-month-old infant.

There is an economic payoff for the government in keeping the program. For every case of retardation prevented in early childhood, there is less money needed later on to pay for special education, institutionalization and foster care. In terms of cost effectiveness, this program might save the government more money in the long run than if school-lunch sizes are reduced. In the case of school kids, hunger and empty stomachs probably would only keep them from concentrating and therefore from learning very much. But if a child is judged in need of nutrition supplements and is denied them due to an arbitrary age cutoff, their brain development is jeopardized.

This is what is worrying people like Judy Wilson, manager of the WIC program in the District. "Addressing the budget reduction through a categorical elimination of age groups of the WIC applicants would just be inappropriate," she sighed.

It is also worrying people like nutritional wizard Dr. Jean Mayer, formerly of Harvard and now of Tufts University. "What worries me is any attack on the WIC program," he said. "The effect on brain development is very early and it has to do with poor nutrition of the mother during preganancy and the nutrition of the children in the first years of life."

Children are what they eat. I don't think this is the prescription Dr. Reagan had in mind when he said the cuts wouldn't harm the poor.