One-third of the 7,500 American children studied in the Know Your Body Program are already on their way to developing cancer, heart disease and strokes, according to American Health Foundation president Dr. Ernst Wynder. Following their parents' lead, they have poor diets and health habits that make them candidates for the diseases that kill two thirds of the adults in this country.

Other findings of the study:

American children are less likely to smoke cigarettes than French and German children. French boys smoke more frequently than French girls, while the opposite is true in Germany and the United States. The heaviest smokers are the children of Kuwait.

Yugoslavian, Greek and American children are the heaviest; those from Kenya, France, Thailand and Nigeria the thinnest.

French and Finnish boys have the highest blood pressure levels, a risk factor in heart disease and stroke. Girls' blood pressures are slightly lower in all countries.

African children have the lowest cholesterol levels and Finnish and Norwegian children the highest. U.S. children rank comparatively low in cholesterol levels.

West German boys are the tallest, followed by New York boys, French boys, Kuwaitis and Kenyans. In most countries, adolescent girls are about one centimeter taller than boys.