Q: My preschooler is very independent and open with strangers--a child with "sand in his shoes," as they used to say about wanderers. He knows some of the consequences of leaving without telling me or without asking permission, but how do I impress on him that age-old problem of not talking to strangers?

Do I scare him with vague references of what could happen if he got into a car with someone he didn't know? Do I make it a game of who he can, or can't, talk with?

I have spoken with his nursery-school teacher about putting something in the curriculum about the matter, but this is more than a school lesson.

How do I explain these dreadful facts of life to my son and still let him know that the world is his oyster?

A: You may be looking at the problem through the wrong end of the telescope.

You want to teach your child to protect his body, rather than fear strangers, and for good reason: 85 percent of all molesters are relatives, friends, teachers, sitters or neighbors. According to a recent study, they're almost always male and their targets usually are female. But the advice is the same for a boy or girl.

Even a 3-year-old should know that his body is his own. Tell your child to avoid any teen-ager or grown-up who kisses, pats or tickles him often, or who wrestles with him a lot--especially when no one is looking. You want your child to watch out for the person who suggests a game and then says he's got to keep it a secret, or who threatens to hurt him if he tells, or says he'll say it isn't so. You're just helping your child realize that he has the right to say no and to listen to his fears.

This is a tough conversation to have with a child, but it is obligatory. According to the National Committee for the Prevention of Child Abuse, 35 states have a "dramatic increase" in reported sex abuse.

How you conduct this conversation is as important as what you say: You want your child to listen well and feel free to ask questions. You'll do this best if you sit beside him and look straight into his eyes the whole time you talk. Use simple words and keep your voice calm and direct, as matter-of-fact as if you were giving the reasons for good nutrition or a sweater on a windy day. Give him time to reflect.You also should be prepared to bring up the subject every few months, so it is commonplace.

If you think this talk would make you too uncomfortable, a simple, read-aloud book, Private Zone by Frances S. Dayee, explains the subject well, if a bit tiresomely. It can be ordered by sending $3.75, which includes postage, to the Charles Franklin Press, 28409 90th Ave., Edmonds, Wash. 98020.

Most of the protection, of course, comes from your vigilance. Parents have to be especially protective with blithe spirits; they're the ones who walk on the far side because they are so sure that all's right with the world.

Even a cautious child isn't necessarily safe from molesters, who otherwise can seem like such ordinary people.

According to Carl M. Rogers, a psychologist and associate director of the child protection division of Children's Hospital, there is "no particular personality profile, life history or other measure" that can predict a future offender, except that he's "almost invariably male."

Although the District Police Department doesn't break down its statistics, the hospital reports that it treats about 250 cases of "indecent liberties" a year, about 25 percent of them boys. They usually are victimized by older teen-agers, while 70 percent of the girls are victimized by young adults between 18 and 30. The average age of the child, says Rogers, is slightly under 9.

Not only should you keep an eye on the teen-agers and adults your child sees, but also keep an eye on your child. He may begin to act differently around someone he's always liked, which would call for some astute observations and some questions for your child. Once a child is sexually molested he may seem afraid of a once-favorite person, or he may disguise his fears with the standard reactions he has to any childhood worry: by getting scared of the dark or of being alone, or he may have nightmares when he goes to sleep.

This child not only fears the future, he feels guilty about the past: He thinks he somehow invited the advances, or at the least, that he should have stopped them. The first idea is encouraged by the molester, the second by his young conscience. He knows something is wrong, but he isn't old enough to do anything about it.

Unfortunately, a child is usually 13, 14 or even 15 before he has internalized his sense of right and wrong well enough to say no when someone else says yes--especially when that someone is much older and perhaps a family friend or relative.

It's almost as hard for a parent to handle the problem. In Florence Rush's powerful book, The Best-Kept Secret (McGraw-Hill, $5.95), sexual abuse is seen as a historically pervasive problem, made all the worse because parents either must deny it--or do something about it. The book is worth reading, if only because it will remind you that most children don't tell anyone when they have been abused. And if they do, their stories are ignored--as too embarrassing or too shameful--or they're not believed (even though children almost always tell the truth about sexual abuse).

That can be the tragedy, if parents deny both comfort and protection when they are needed most.

Some may think a preschooler is too young to be told about the seamy part of life, but if you deprive a child of a little innocence now, he may not lose all of it traumatically.