When Anne Lindbergh was a child, she and her siblings liked making up stories that began "What if . . ."

"What if you were in bed and someone came in the window and offered you three wishes?" Lindbergh asked a large group of children at the Cheshire Cat Book Store on Connecticut Avenue. "My sister and I always used to argue about whether it would be fair to use one of the three wishes to ask for three more wishes."

Now 42 and the author of several children's books, Lindbergh is at the children's book store to autograph her latest -- The People in Pineapple Place, a fantasy set in Georgetown -- and to launch the store's annual spring writing contest. Lindbergh has written the beginning of a short story, and children six and up are invited to write the rest -- in three pages or less.

"I had so much fun writing this paragraph, I wanted to go on," says Lindbergh, a slender, fresh-faced blond woman wearing slacks, a silk blouse and an Afghan vest. "I like having a magic object. Have you read The Lion, The Witch and the Wardrobe? In that, the characters go into a magic cupboard. I thought I'd use a flashlight. It turns on and off and gets you into different places.

"This isn't like a test. There's no real answer. I don't have a finished story in my head. The nice thing about writing children's books is that you can make your own rules."

Lindbergh, who enjoys re-reading such favorite children's books as Frances Hodgson Burnett's The Little Princess, is the daughter of Charles and Anne Morrow Lindbergh, both writers.

"My mother didn't write children's books," she says between autographing chores. "If she had, I wouldn't have used the name Lindbergh in my writing. At first I wasn't going to, but my father was hurt at the idea. My grandmother, Elizabeth Morrow, did write a children's book, called My Favorite Age. When we were little, we were always asking my mother to 'tell us about when you were a little girl.' And my father used to make up stories, mainly about animals."

For her own stories, Lindbergh gets a lot of ideas from her own children -- ages 16, 11 and five.

"When we first moved here, my son Charles was nine," she recalls. "We were walking around and came upon Pomander Walk. He remarked that it looked like it came here out of another time. I remembered that later, and Pomander Walk -- which is an alley off Volta Place -- inspired Pineapple Place. I had a lot of fun doing research for the book. For the roller-skating scene I went around to different buildings on the Mall to see which would be the most outrageous place to roller skate. When I found it -- the dome of the west building of the National Gallery -- I stood there for so long that a guard came up and asked if I needed help. I said no -- that I was just looking for a good place to roller skate. He was horrified. 'Oh no,' he said. 'It's not allowed.'

"And there's one part when the Pineapple Place people go back to 1939. I did a lot of interviewing to find out what Georgetown was like in 1939. There was an old couple who used to come out and sit under their holly tree and talk. She was telling me about the trolleys that used to go by and mentioned their bells. Her husband said, 'Nonsense, the trolleys didn't have bells.' They had a big fight about it, and that fight is going into my next book." FINISHING THE FANTASY

The Cheshire Cat Book Store Spring Writing Contest is open to children six and up. Contestants 10 and older will be judged in a separate category. Write or type your ending to the beginning printed below in three pages or less. Don't recopy the beginning. Entries must reach the Cheshire Cat Book Store, 5512 Connecticut Avenue NW by 5:30 Saturday, April 2. Winners -- one in each age category -- will be announced April 9. Each will receive a gift certificate and an autographed copy of The People of Pineapple Place. Contestants may pick up their entries after the contest and receive a certificate of participation. Here's the beginning of the story: "Why does it always have to rain on Sundays?" I asked. Nobody answered. My mother and father were reading the paper and my brother Ben was staring out the window. "Come on, Ben!" I said, "Let's go play in the clubhouse." Our clubhouse was just a big closet that we had fixed up with posters. It didn't have any windows and the lightbulb had burned out the day before. "Did anyone remember to buy a new bulb?" Ben asked. "Use a flashlight," said my father. "There's one on the kitchen counter." Ben went to get it and came back looking puzzled. "How does this thing work?" he asked. "There's no button to turn it on." "Oh, for heaven's sake!" my mother groaned. "Use your brains! Tap it, or twist it or if that doesn't work, just wish! But could we please read the paper in peace?" Ben and I went into the clubhouse and locked the door. "It's pitch dark in here!" I said. "Hurry up and turn on that flashlight." "I can't," said Ben. "I tapped it and twisted it and it still doesn't work." "Then wish!" I said, mimicking my mother. Ben laughed. "If I had a wish, I wouldn't waste it on a flashlight. I'd wish I were invisible, or something like that." As he said the words, the flashlight suddenly turned on. It lay shining in the middle of the closet floor and Ben was gone. "Ha, ha, very funny," I said. For a moment I thought Ben had slipped outside while the light was off. But the door was locked with the key still in the keyhole -- on our side!!