WASHINGTON (AP) _ A private environmental group contends in a new report that preschool children are consuming cancer-causing chemicals in fruits and vegetables at levels that expose them to health risks many times greater than the government considers safe.

The findings by the Natural Resources Defense Council were immediately challenged by the Environmental Protection Agency, which said in a statement Friday that health risks to children are taken into account when setting allowable chemical residue levels.

The Natural Resources Defense Council, a group specializing in environmental and health matters that has criticized government policy in numerous areas, is scheduled to announce its findings at a news conference Monday. An executive summary of the report was obtained Friday by The Associated Press.

Janet Hathaway, one of the authors, declined to comment on the two-year study except to say, ''Our work definitively shows there is a greatly increased risk to children from chemicals in their diets.''

The study, according to the summary, concludes that preschoolers are at an increased risk because they consume many times more in fruits and vegetables than adults relative to their weight and are more vulnerable to the effects of toxic chemicals.

The summary said the authors had concluded, after analyzing consumption data and allowable residues of eight carcinogenic chemicals on 27 fruits and vegetables, that as many as 5,500 to 6,200 youngsters among the current population of preschoolers ''may eventually get cancer solely as a result of their exposure (to the chemicals) before six years of age.''

This represents, the report continued, a risk of at least five people in 20,000 getting cancer because of their exposure before the age of 6 years, much higher than the one in one million considered allowable by the EPA in its risk analysis formula.

Critics of the study said it raises false fears among parents.

''It's voodoo statistics. This has nothing to do with the realities of disease,'' said Dr. Elizabeth Whelan, executive director of the American Council on Science and Health.

There ''is no evidence of even one case of human cancer in children and adults being linked to exposure to the minute pesticide residues in food,'' said Dr. Whelan in a telephone interview. She maintained there are greater health risks in not using the pesticides.

The environmental group's study concludes that the typical preschooler receives four times greater exposure on average than adult women to eight carcinogenic pesticides the group evaluated and that in some cases the increased exposure is as much as 18 times that of adults.

''Our nation's children are being harmed by the very fruits and vegetables we tell them will make them grow up healthy and strong,'' concludes the report summary.

The group's conclusions were based on fruit and vegetable consumption figures from the Agriculture Department and on levels of pesticide residues found on 27 fruits and vegetables according to the Food and Drug Administration and the EPA, the study said.

Preschoolers eat six times as much fruit in general, seven times as much grapes, apples and apple sauce and 18 times as much apple juice as an adult, the study said. It said that half of all fruits sampled by the FDA in 1987 ''had detectable levels of pesticides.''

In a statement, the EPA said the agency ''is concerned about potentially higher pesticide exposure to children and infants,'' but that it takes those concerns into account when determining levels of pesticides that are allowed on food.

John Moore, acting EPA deputy administrator, said in an interview, however, that the agency does not agree with some of its critics that it should across the board ''add another potency factor in addition to what we use to accommodate the fact that kids are exposed more'' to certain foods containing some level of pesticides.

He added that the agency's chemical tolerance levels for fruits and vegetables ''is designed to protect the average person against any short or long-term harmful effects that might result from an entire lifetime of exposure to pesticide residues in food, including exposure during childhood.''

The private group's study saw exposure to daminozide, a growth regulator, as posing the greatest risk to children among the carcinogenic chemicals they examined with a potential cancer risk to preschoolers of 1 case in every 4,200 people exposed. The EPA recently said it plans to order the product, which is used on apples, off the market and the producers of processed apple products have said for the most part it is no longer is being used by their industry.