Good dental care and decay-free teeth begin at home, pediatric dentists say. Children should be taught how to brush their own teeth by the time all their baby teeth are in at about 2 1/2 years old and parents should have their children visit a dentist by that time.

''When I speak to parents, I tell them to make sure their children brush their teeth and use dental floss after each meal,'' said Micheal Roberts of the National Institute of Dental Research.

''But I'm also a realist. Only a fractionof people continue to floss after their initial enthusiasm wears off. And children cannot brush their teeth to health. There are too many nooks and crannies where food can be trapped.''

So Roberts ends up stressing the importance of diet. ''I know kids will get sweets,'' he said, ''but parents can eliminate particularly troublesome foods.''

The basic idea is to cut down on sugary foods and to be particularly wary of sweet foods that are sticky. ''I personally think that no matter how healthy you are, when you have sugar, you take a calculated risk of decay,'' Roberts said, ''Very few people are inherently resistant to decay.'' The biggest problem is with sweet snacks because when children eat sugary snacks they are even less likely to brush their teeth afterwards than if they eat sweets after a meal.

Peanut butter mixed with sweets like jelly or banana can also be a problem, Roberts said, since the mixtures sticks like glue to the grooves of childrens' teeth. As an alternative, he proposes old-fashioned peanut butter, with no sugar added, eaten from a spoon or spread on crackers. Other healthful snacks are fresh fruit and cheese.

And when you take your child to the dentist, Roberts said, try not to communicate any apprehensions you may have. ''Basically, tell the child as little as possible,'' he said. ''Just say you are going to the dentist. Don't make it sound like a big deal.'' The pediatric dentist will take it from there.

Other advice from the National Institute of Dental Research:

*Ask your dentist or area health department if your community has enough fluoride in the water. If not, about fluoride drops or tablets. In the Washington area, virtually all water is fluoridated.

*Avoid creating a ''sweet tooth'' in your baby. Don't sweeten cereal or fruit to courage the child to eat. When they're older, they'll be happy with slices of apple and other raw fruits and vegetables if they haven't learned to eat candy, cookies and cakes.

*Never put a baby to sleep with a bottle of milk or juice. During sleep, this liquid pools behind the baby's teeth and keeps them bathed in sugar for hours which leads to decay. If your baby needs a bottle to go to sleep, use a bottle of water.

*Make it a habit to take your child to the dentist about every six months.