His face is one any mother would love for her child's pediatrician: a combination of Walter Cronkite, John Lindsay and Marcus Welby.

Dr. Lendon Smith, better known as "The Children's Doctor" for his TV show and book by that name, is trading on his wise and witty celebrity image to teach parents that "youngsters act what they eat."

Proper nutrition can cure everything from acne to drug abuse, bed-wetting and crime, contends Smith in his latest book, "Feed Your Kids Rights."

Although medical school taught the pediatrician "that Americans were the best-fed people on earth and that only quacks used nutrition in the treatment of disease," he began to link diet with behavior after noticing that a majority of hyperactive children he treated ate "gallons of ice cream" and other sugar-charged foods. Often their mothers were plump and their fathers loved a "seven-course dinner: a six-pack and a piece of pie."

A proper diet, eliminating sugar and white flour, will calm most hyperactive children, says Smith, who prescribes drugs only as a last resort.

"If school authorities want to stop discipline problems and vandalism in the classroom, they must do away with sugar and junks foods in the halls and close the candy stores within two miles of the school," he says.

He calims that 75 percent of people in prison today were hyperactive as children and that drug addiction is a small step from chocolate, coffee or cola addiction.

"You've got to assume a lot of kids act antisocial because they feel rotten. "If kids don't feel good, and have headaches from eating junk food, they may put shoe polish in their veins or do anything to make themselves feel better."

"The Prevention Diet" is Smith's name for eating habits he believes everyone should follow as a lifetime program for good health.

His advice:

Avoid the "anti-nutrients": white and brown sugar, corn and cane syrup, maple syrup, molasses and honey, commercial ice cream, boxed cereal and white flour.

Shun as many commercial products as possible, especially those that are stabilized, emulsified, added to and processed.

Eat natural foods four to six times a day in small amounts. Eat meals off a small plate, a coffee saucer for children or a bread plate for adults.

Start your child's day with a breakfast of oatmeal, whole-grain bread with old-fashioned (unsweetened) peanut butter, lentil soup or stew, foods whose calories are absorbed slowly.

Pack a lunchbox with an avocado and syrup sandwich, raw vegetables or Smith's own favorite sandwich: peanut butter and cheese on whole wheat bread.

Stock raw vegetables in the refrigerator and stuff kids' pockets with raisins and nuts so they can nibble nutritiuos food when they get hungry. Nibbling on protein foods and complex carbohydrates (i.e., vegetables), according to Smith, keeps the blood sugar from dropping into the range in which quick sugar is craved. "If family members nibble all day by the time they sit down to supper their blood sugar will allow for a pleasant social interchange, instead of the usual snarls and elbowing that accompany feeding time at the zoo."

Supplement the diet with daily vitamins and minerals.

"Read your kids." For example, Smith says, acne or white spots in the nails could signal a zinc deficiency, inability to get out of bed in the morning or a poor sense of humor a lack of B vitamins, rhythmical tension-relieving action (such as foot-tapping or rocking) a low calcium level, and reading problems insufficient "memory vitamin," or B6.

"Feed Your Kids Right" is published by McGraw-Hill (250 pages, $9.95). CAPTION: Picture, Dr. Lendon Smith