One Monday Morning, by Uri Shulevitz (Scribner's $2.95).A small boy is out when royal visitors come to call. They return, but he's out again. Suspense mounts. Will he ever meet the king and his retinue? Handsome, amusing, easy to read. (Ages 3-5)

Barbapapa's New House and Barbapapa's School, by Annette Tison and Talus Taylor (Scholastic, $1.95). First time here in paperback for these popular, good-humored and endearing characters. The Barbapapa clan look rather like garden squashes but they can assume any form they want.(Ages 3-6)

Dick Whittington: A Story from England; The Lady of Staveoren: A Story from Holland; Matt the Gooseherd: A Story from Hungary; The Mouse King: A Story from Tibet; (Puffin Folktales of the World, $1.95 each). The first four in a series produced under the auspices of International P.E.N., with different storytellers and artists for each one. Only The Mouse King is a satisfactory creation - with a clear text, charmingly appropriate pictures, and a bit of Tibetan lore at the end. (Ages 5-9)

The Dutchess Bakes a Cake, by Virginia Kahl (Scribner's, $2.95). Long ago and far away, before instant mixes, aristocratic ladies had limited cooking skills. In this case, however, a duchess makes up for her ignorance with her enthusiasm. Told in rhyme, accompanied by plain, funny pictures. (Ages 5-8)

Marie Louise's Heyday, by Natalie Savage Carlson. Illustrated by Jose Aruego and Ariane Dewey (Scribner's, $2.95). A young mongoose gives up a day she anticipated having to herself and valiantly babysits for five impish possums. Droll, with stylish art. (Ages 5-8)

The Shrinking of Treehorn, by Florence Parry Heide, Illustrated by Edward Gorey (Dell Yearling, $.95). At first his parents don't believe him when Treehorn points out that he is getting small. Goreyhs sinister line suits Heide's deadpan delivery; sophisticated children will enjoy this homage to Alice . (Ages 5-8)

What? A Riddle Book, by Jane Sarnoff and Reynold Ruffins (Scribner's, $3.95). Well designed, arranged thematically, gauranteed to generate giggles and guffaws. For instance: Why can't a train sit down? Because it has a tender behind. (Ages 5-up)

The Courage of Sarah Noble, by Alice Dalgliesh. Illustrated by Leonard Weisgard (Scribner's, $2.95). In colonial Connecticut, Sarah Noble is left by her father for a short while with the Indians. The young heroine lives up to her name; readers, meanwhile, will learn about mutual trust and bravery. (Ages 6-10)

The Key Word and Other Mysteries, by Isaac Asimov. Illustrated by Rod Burke (Avon Camelot, $1.25). Five short, easy-to-read mysteries in which a detective's son gets in on the act. (Ages 7-10)

The Zoo Conspiracy, by Betty Levin. Illustrated by Marian Parry (Avon Camelot, $1.50). A zoo's animal community is stirred up by a malicious adolescent who whispers that a new acquisition will be more rare (and thus get more attention) than any of them. The creatures' personalities are brilliantly conceived; a wonderfully amusing - and informative - fable. (Ages 8-12)

Arilla Sun Down, by Virginia Hamilton (Dell Laurel Leaf, $1.50). A young girl - part black, part Indian - talks about herself and her relationship to the world around her in this compelling story by the award-winning author of M.C. Higgins the Great. (Ages 10-up)

The Other Way Round, by Judith Kerr (Dell Laurel Leaf, $1.50). Anna and her family, refugees from Nazi Germany (first met in When Hitler Stole Pink Rabbits ), are now in London, enduring the war alongside the British. A richly developed, sensitive novel. (Ages 12-up)

The Nightmares of Geranium Street, by Susan Shreve (Avon, $1.50). The Nightmares are a tame neighborhood "gang" who become fixated on the mysterious activities of a new member's aunt. Fast-moving and true-to-life about kids' boredom and curiosity. (Ages 12-up)

Sandy: The Autobiography of a Star, as told to William Berloni and Allison Thomas (Wanderer, $3.95). Move over, Old Yeller, Rin Tin Tin, and Asta, for the only pooch who can howl to "Tomorrow." (Ages 12-up)

Basic Beginings: A Handbook of Learning Games and Activities for Young Children, by Audrey Burie Kirchner (Acropolis, $12.95). Things to do, be, see, hear, touch, draw, cut out, smell, count, glue, etcetra. Iths a book for professional and concerned adults, the purpose of which is to encourage "loving to learn."