Children's fantasy; A land where lost things are found; The Planet of Lost Things, by Mark Strand, illustrations by William Pene Du Bois. New York: Clarkson N. Potter. 28 pp. $9.95.

If counting sheep doesn't send a child right off to sleep at bedtime, try giving him or her a question to think about. For instance, ''If a tree falls in a forest and no one is there to hear it fall, does it make a sound?''

Mark Strand, a prominent American poet-turned-children's-author, starts the story of a little boy's dream with this question. When the boy, Luke, does fall into dreamland, he takes off via spaceship to a planet where everything in the world that has ever been lost eventually ends up. Naturally the first thing he hears upon landing is the tremendous sound of a tree crashing to the ground. He then discovers piles and piles of such things as lost letters; those many lost socks, hats, and mittens; and even a tree full of lost cats surrounded by a pack of lost, barking dogs.

Although children will enjoy the story's imaginative exaggerations, issues for family discussion are introduced: Luke first learns about extinction when his friends, The Missing Person and The Unknown Soldier, point out a group of dinosaurs and auks (a good vocabulary word) who live on a plain on this unusual planet. Author-poet Strand offers vivid images like a ''curtain of mist'' and ''a place where the ground seemed strewn with diamonds'' that are ''only the dew that goes away when the sun comes up.''

The words of this most imaginative story are picturesque enough to hold a child's attention, but the illustrations by Caldecott Honor and Newberry Medal winner William Pene Du Bois promise hours of enjoyment. (Ages 5 to 8).