Grandpa changes roles with the kids; What's Under My Bed?, by James Stevenson. New York: Greenwillow Books. 30 pp.

''It was awful! A creature with wild hair, no head, and a long tail was standing there.'' ''But, Grandpa,'' said Mary Ann, ''wasn't that your shoes and your bathrobe and your hairbrush, just where you had left them?''

''Why, that's just what it was,'' said Grandpa.

Thus proceeds a confidence-inspiring exchange of roles by which Grandpa, eager to quell his visiting grandchildren's nighttime fears, gently elicits their assurances as he relates a ''scary nighttime'' ordeal he had when he was a boy. One by one his own bedtime imaginings are seen by the kids for what they really were, and laid to rest through a liberating common-sense analysis - everything from pirates fighting with swords (cats jumping on garbage cans), to wailing ghosts at the window (high winds whistling through the cracks).

Children will love this book's humor and cartoonlike watercolors. Mr. Stevenson's honest and compassionate treatment of bedtime fears represents a rational and sensitive approach to what can be a serious challenge for some children. Some books addressing this subject induce the very fears they seek to debunk. And parents will have to first consider their own children's responses in order to gauge the impact of this book's many nightmare images. Yet Mr. Stevenson's intent is certainly to liberate children from the meanderings of dark thoughts - and no doubt many children will derive just that benefit from it.