TO WRITE a convincing fairy tale is not an easy task. It is even more difficult in this age of disbelief than in the time of Grimm and Andersen. Schiller argued that the fairy tale is actually a poetic form, and therefore it demands unusual care in its construction. Lloyd Alexander, Richard Kennedy and Jane Yolen have all produced admirable recent experiments with folk material. Now Walter Wangerin Jr., author of the justly honored The Book of the Dun Cow, explores the form in Thistle.

Here he tills familiar ground. A potato farmer and his wife long for children and are eventually blessed with four: Pine, tall, slim and handsome; Oak, strong and bold; Rose, pretty and frail; and Thistle, short, stubby and a terrible cry-baby. One day their father digs up a monstrous tuber, a rapacious Pudge that immediately eats up the farmer and his wife. Each of the children in turn seeks some weapon to fight the thing, rebukes a witch's kisses, is cursed and consequently eaten up by the Pudge. That is, all except Thistle: Too overcome with grief over the loss of her family, she permits the witch to kiss her, and is rewarded with a thorn everywhere the woman's lips touch her body. Now, when the Pudge swallows this child, it explodes into mashed potato, releasing all the others whole. And they make soup of the creature's remains.

It is no betrayal of the plot to reveal the punch line. Thistle is that same old story: The least saves the greatest. It might have been an amusing variant of an overused theme were it not told in such stiff, pseudo-archaic language ("And, while Thistle wept in worry on account of him, he went away," "Six were four, for the loss of two parents"). And why is the witch the only one to use the old familiar pronouns "thee" and "thou"? Be she a Quaker? Too many questions are left unanswered by the text. What horrible violation of the earth has produced the Pudge? Earlier the farmer was in harmony with his god and the land. And what became of his tools? He must have had more than just a hoe to work the fields. These could have served as weapons to fight the Pudge. How can the three older children be so stupid to believe that by taking root they will be invincible to the Pudge? Why cannot Rose sprout thorns when she takes root? And who ever makes soup out of mashed potato?

Obviously the author has not thought his fable through. Even a fairy tale requires some internal logic, at least some moral order to its universe. The jacket explains that the story was created to console Wangerin's youngest child who had been cruelly teased by her brothers and sister for crying. However, what may have been soothing in the telling by a well-meaning parent is only flat on the page.

The story provides Marcia Sewall little action on which to devise a full-blooded picture book. She is defeated in the tedious quests of the three older children. While her Pudge is ingenious, her drooling witch is merely repulsive. And who chose that inappropriate jacket illustration? Thistle disappoints, because one expects better from both Wangerin and Sewall.