Q. Last week Susie, our 13-year-old, gave a slumber party, which meant that her 12-year-old brother had to sleep downstairs so the girls could have his room, too. That's where the trouble began.

They were choosing beds when they began to argue: No one wanted Billy's. The mattress was too lumpy, they said, and it was - until Susie fished around and found that her brother had stuffed 16 copies of Playboy, Penthouse and Hustler magazines between the mattress and the box spring.

You could have heard the kids a block away, chasing Billy up and down stairs, in and out of the house, shrieking and waving those magazines. He embarrassed Susie (and himself), and he upset his father and me a good deal. We live in a churchy neighborhood in Alexandria, for one thing, and besides, Billy is much too young for this. I can't imagine why he would want to read this trash.

A. It seems that Billy's imagination is working better than yours, but you needn't worry much about the reading. It's the ogling that interests him most. The lure of the lurid is still so new and wonderful he barely skims the text. Have you ever heard of a college freshman who drinks slow enough to savor a glass of wine? Not likely.

A wholesome boy of 12 may play soccer every day and earn merit badges regularly, but that doesn't mean he isn't interested in sex.

This interest may start a little later (or a little earlier), but when a gaggle of girls passes by, you can bet that the boys, as indifferent as they seem, are just as conscious of those mosquito bites and training bras as the girls. In fact, you could worry if your child isn't staring at those T-shirts with X-ray eyes.

You can't protect him from the scenery of the '70s, and that includes soft porn. If nothing else, he would be the only one in the neighborhood who had never viewed a frontal view.

This is not to say that your child should keep a library of skin-flick magazines by his bed. His room may be his castle, but unless it has a drawbridge for a door, there is going to be considerable traffic going in and out. If these magazines bother anyone in the family - and they probably will - they should be kept out of sight, or even out of the house depending upon how strongly the feelings run.

If you do allow them, the matter of safekeeping is important - and after one slumber party, we're sure your son would agree. Permission to keep them on a high closet shelf is much better - and more comfortable - than a secret pile under the mattress. Besides, you want to encourage discretion, not furtiveness.

The kinds of magazines, and the numbers, are another question. Most parents would think 16 issues are a bit much, especially for a 12-year-old, while a moderate parent might permit two or three Playboys, some Penthouses in a few years and Hustler: Never.

Personally, we are more disturbed by the subliminal messages these magazines send than the graphics. According to them, sex is no more than a mating game, a technical exercise where men and women use each other like toys - hardly the values you want your child to have. This is something you will want to talk with him about, not as a preacher, but as one human being to another.

Whatever decision you make, your own instincts are the ones to be followed. This is, after all, your house, you pay the bills, and you have the right (and duty) to draw your own boundaries. No matter that they are different from the boundaries drawn by other parents. You are rearing your children, not theirs. Any time you live by someone else's rules you lose a little bit of your own integrity and no one will be more aware of that than your child. If you can't be true to yourself, how can he? .

If you allow Billy to have a few of the less offensive magazines - no matter what the neighbors might say - we think you would be wise, and much appreciated. Every child has needs, even if we can't understand them.

The child who knows that you respect his feelings will do his best to respect yours.