"WHY," SAID MY FRIEND Bobbie after a pregnant pause over the telephone, "are you doing this? Don't you realize that now all the rest of them are going to want to do the same thing?" The thing, in this particular case, was what used to be known as a slumber party. In the elementary school set, however, it is known as a sleep-over. Whether I didn't know what a sleep-over was or whether I wasn't listening when the request was first submitted a year ago is a detail lost to the ages. In any event, I found my about-to-be-six-year-old fixing me with a steely look several weeks ago and reminding me that I had promised that he could have one for his birthday. Looks may not kill, but they can certainly keep you from reneging.

The guest list was trimmed to six of his most intimate friends. "You're crazy," said one mother. "You're really going to do it?" said another. "I'll remember this," warned Bobbie.

The optimistic approach to multiple children is that the first child gives you unlimited opportunities to learn and the second child gives you unlimited opportunities to learn from your mistakes. My first son, for example, had a slumber party for his sixth or seventh birthday that left us vowing a number of things, among them never to have another slumber party. We had expected them to be noisy and we had expected them not to sleep. We had not, however, expected them to show up with a variety of acute personal problems. One had a terrible case of gas. Another was mortally afraid of the dark. Another had an accident. And another informed us over dinner that he had been an accident. By dessert, he had spilled the beans on his family's most embarrassing secrets, leaving me with the firm resolve never to let my kid out of the house.

But time is a great healer. Nothing, I cheerfully reassured my husband, who doesn't heal as fast as I do, could be as bad as the first slumber party. "All right," he said, "It's your ball and bat." It's your ball and bat is a sports term that he utters in the same tone of voice that my mother reserved for "You've made your bed; now you're going to have to lie in it."

The game plan called for dinner, followed by movies rented from the library, followed by bedtime. What could go wrong? Besides, there were four big people on hand for the evening and if a first-grade teacher can handle 25 of them for a day, the four of us could handle seven of them for the night.

Shortly after the children arrived, it became clear that the secret to survival that evening was to entertain them and under no circumstances to allow them to entertain themselves. By 9:30, they were tucked angelically in their sleeping bags on the floor of the family room while my son the 15-year-old began showing the last of the cartoons.

But it turned out that we were not the only family that had been planning a party last Saturday evening. The belle of the neighborhood was celebrating her 18th birthday. With a toga party. "It might be wise to move your cars into your driveways," advised the neighborly note she sent out Friday. By the time the children were ready for bed, it became clear that this was good advice. A toga party, it turns out, is not merely a party where teen-agers get dressed up in sheets and get drunk and take leave of whatever senses they have. It is also a party in which they arrive in cars, preferably without mufflers, which they then drive up and down the block at varying rates of speed.

By 11:30, the birthday boy appeared in the living room, rubbing his eyes and informing us that he couldn't sleep because of the car noises. I crept into the family room. There in the darkness was one sleeping child and six wide-awake ones, toy animals in the arms of some and pillow fights in the eyes of all. This called for a creative solution. "C'mon," I said, motioning them to the window facing the street. "Climb up here on the sofa and you can watch the teen-agers!"

By then, the teen-agers were putting on quite a show. Some were perched on top of cars, others were wandering about drinking beer and hooting, and still others were baptizing the bushes. We spotted one in a neighbor's driveway. So did our neighbor, who to the children's delight, appeared in the driveway in his pajamas. Then, as his appreciative audience juggled for position on the sofa, he reappeared in his day clothes. For his finale, he went back into his house and reappeared in yet another costume, this time a coat and a hat, which provoked a round of giggles, and which he kept on until he abandoned his post around 12:30 a.m.

By the time the toga party ended, the children were imitating teen-agers, and I was imitating the wicked witch. I ordered them into their sleeping bags. I threatened to call their parents and take them home. Finally, in desperation, I asked them if they wanted a lullaby. Suddenly, they were silent. Then, very softly, one little voice said yes, and five other little voices agreed.

It may not have been Brahms, but it worked.