Q.: I have a 6-year-old son who plays regularly with a little neighbor girl a few months younger than he. During the past six months, this girl's mother (my good friend) has told me of about four or five instances of sex play between the two of them.

First, my son asked this little girl to take off her underwear. There was a second instance of this sort, and then there was some discussion of "bottoms," I think. Finally, the girl's mother found the two of them in the bathroom while her daughter was preparing for her bath and was undressed. They were both embarrassed when they were found.

After the first incident, my friend and I spoke to our children about privacy and respecting one's body, etc. The second two incidents were very minor and were ignored. However, after the last incident, the girl's father was very upset and she was sent to her room.

I was concerned about it too, and I had a long discussion with my son, eliciting his promise not to violate the girl's privacy again. I was concerned, too, about the implication that my little boy was taking advantage of his playmate.

The next day my friend and I had a long discussion about our handling of the situation. Both of us felt inadequate and yet neither of us wants our children to think of themselves -- or sex -- as dirty or evil.

I know that this curiosity is natural and normal, but it's still a sensitive subject. What should we have said? At what point does behavior like this become abnormal? What should we say or do if it happens again?

A: If this kind of behavior were abnormal, all the 6-year-olds in the world would be abnormal. The explorations you describe would only be a problem if they were blatant and much more frequent.

The behavior of your child and his friend is commonplace. Whether parents know it or not, there is a certain amount of groping at 6 (and 5 and 4). Children are born to be curious about everything, including sex. And they're born to enjoy it, because that's how nature keeps the human race going.

Scenes about their sex play will shame them, but they won't stop it. Supervision will, however -- most of the time. Your child and his friend need a few rules when they're together. It's a good idea to:

Keep the door open if they're playing together in a room.

Allow only one child in the bathroom at a time.

Have them keep their clothes on.

Time will also help. By 7, boys and girls separate instinctively by gender, which makes them immediately lose interest in doctor games and other explorations. Even though boys and girls are intensely interested in each other, they seldom play together and they certainly won't undress in front of each other. Modesty is very important at 7.

They still may joke on the playground about bottoms -- in fact, they surely will -- but the talk will usually be scatalogical, not sexual, because that's what children understand.

There is a strange misunderstanding abroad today. Some people seem to think that if young children can learn to read and write and express themselves so well, they can understand all there is to know about sex, and book after book is produced to tell the child of 4, 5, 6 or 7 all about it. Some are mighty explicit, but none suggest that parents ask the child what he has learned. The parents (and the publishers) might be surprised if they did.

Several years ago a group of bright middle- and upper-class California children, all taught about sex by their well-educated, modern parents, gave their answers, which showed that children understand about sex the same way they understand anything -- according to the level of their thinking.

Generally, the 3's and 4's, because they couldn't imagine a world in which they didn't exist, thought a child had always lived someplace -- perhaps in some other mommy's tummy. At 5 or 6, they thought the baby was bought in a store or grew from a seed that was swallowed.

The child of 7 or 8 began to understand the father's role, but he was at least 8 and even 12 before he really understood cause and effect, past and future, and still his ideas were hazy. Most 11- and 12-year-olds said either the egg or the sperm was a miniature person. Abstract thinking is beyond most children until adolescence. That's why a full course of sex ed for the very young is a waste.

This doesn't, however, mean you ignore the subject. A child should be given lectures on the sanctity of the body, as you and your friend have so wisely done, and should be taught about sex in as much detail as he can absorb. These talks will never be completely easy for you. No matter how comfortable we are when we explain sex to other children, we stumble when we talk about it to our own.

You could leave this job to the school and the library, but parents are the only ones who can emphasize values so well. Nevertheless, it will be years before your child -- or your neighbor's child -- is going to be able to live by those values on their own, day after day. Children can't count on their conscience completely until they can think in abstractions well enough to consider the consequences and be so independent they will take responsibility for them.