The last thing a slumber party is about is sleep. It is about giggling till dawn, building tent cities in the living room, having pillow fights and popping popcorn. It is what a Washington child psychologist calls "a rite of passage, children's first real move out of the family setting."

And whether it is a happy passage or a disaster can depend on how it's handled.

"Parents should make the decision about when the child is old enough to have a slumber party--not let themselves be pushed into it by the child," says the psychologist.

"In general, if parents ask me, I say that when children want a slumber party, tell them they can have one on their next birthday. There is usually that long a lag between their first asking and actually being ready for it."

A mother of two girls says that the only rule she lays down is how many they can invite: usually 6 to 8.

"'Less than that and it isn't a party. More and it's impossible. And I let the girls manage everything. As far as food, all rules are off. They can choose whatever junk food they want. They usually order a pizza or make hamburgers and french fries. And they do it themselves, setting the table, cooking the meal, clearing up.

"It's a lot of fun seeing your children with their friends and watching them be hostesses. I leave them alone unless I hear great screams or the record player is too loud. Then I go in and ask them to quiet down."

"I find that although boys are supposed to be rowdier, it is the girls who have the occasional quarrel," says the mother of twins.

"The boys just go out and sleep in their tents in the backyard, or, if it's raining, they drape tents over the chairs. The boys always seem to wind up in tents. I usually try to have a treasure hunt to give them something to focus on, and then feed them fish and chips in newspaper cones."

"Food is the major thing," says the mother of a 6-year-old.

"My daughter and her friends have all grown up with Adele Davis-type, health-food mothers. Therefore, naturally, they adore anything that comes in a package, particularly if it's frozen. They always want pizza, but they never want the kind of pizza that comes from a neighborhood carryout. They want Stouffer's frozen french bread pizza. I think they respond to the picture on the box.

"Children are not good at late dinners, so I ask the parents to bring them between 6:30 and 7 p.m. That way they can eat, play a few games and giggle and whisper for hours. The whole night is going to be a loss and you should assume that no one will get any sleep.

"I think the key to making the party successful is to be sure all the children are picked up by 10 a.m. the next morning. Children can't sustain that kind of excitement much longer. They're tired and starting to get cranky. It's best to give them breakfast--frozen waffles, of course--and send them home."

"I never have more than six children," says another mother who agrees with the others and with the psychologist that six to eight is the optimum number for a slumber party.

"That way I can spot anyone who needs special attention and see that they get it. Once or twice I've had a situation where the child had something bad happen at school that day, or it was their first experience away from home. With a small group you can spot the problem and try to take care of it, or, if that doesn't work, see that the child gets home."

The psychologist suggests that to know what's going on without acting like a guard, parents look in every so often with a legitimate errand--announcing that there's popcorn or ice cream available, for instance. He also suggests arranging things so that the more exuberant activities--pillow fights and games--take place early in the evening. Later, show a movie or let the children play favorite records to quiet them down.

It's a good idea, says the psychologist, to choose films a bit more sophisticated than the children would ordinarily see. Part of the purpose of the slumber party is to make them feel older, to make them feel that after seven or eight trying years of being told what to do, they're finally getting out on their own.