Recipients of the top two 1982 US prizes for children's books were announced last week in San Antonio. Winner of the Caldecott Medal, awarded annually for the most distinguised American picture book is ''Shadow'' (Charles Scribner's Sons), illustrated and translated from the French by Marcia Brown, who also won the Caldecott in 1955 for ''Cinderella'' and in 1962 for ''Once a Mouse . . .''
Two runners-up, known as Caldecott Honor Books, were also designated. They are:
* ''When I Was Young in the Mountains'' (E. P. Dutton), written by Cynthia Rylant and illustrated by Diane Goode.
* ''A Chair for My Mother'' (Greenwillow Books), written and illustrated by Vera B. Williams.
Also announced was the recipient of the John Newbery Medal, given yearly for the most disinguished contribution to children's literature in America. The winner was ''Dicey's Song'' (Atheneum), by Cynthia Voigt.
Five Newbery Honor Books were also named:
* ''The Blue Sword'' (Greenwillow Books), by Robin McKinley.
* ''Dr. De Soto'' (Farrar, Straus & Giroux), by William Steig.
* ''Graven Images'' (Harper & Row), by Paul Fleischman, illustrated by Andrew Glass.
* ''Homesick: My Own Story'' (G.P. Putnam's Sons), by Jean Fritz, illustrated by Margot Tomes.
* ''Sweet Whispers, Brother Rush'' (Philomel Books), by Virginia Hamilton.
The awards were announced at the close of the American Library Association (ALA) midwinter conference, Jan. 8-10. A 15-member selection committee comprised of children's librarians, library-science professors, and a Horn Book editor made the Caldecott selection from the roughly 900 children's picture books published in the United States during 1982.
A similar committee selected the Newbery winners from over 2,000 children's books published last year.
Caldecott committee members examined the books throughout the year, looking for outstanding art technique and excellence of artistic interpretation. Through meetings and phone calls they shared views and discussed front-runners. The jury narrowed the field to 100 finalists.
The voting process is the same each year. When the selection committee meets at the ALA's midwinter meeting, the finalists are placed on a long table and gradually eliminated by successive votes. On each ballot jurors indicate their first, second, and third choices - firsts getting four points, seconds three, and thirds two. A winner is decided when the book with the most votes leads its closest competitor by at least eight points.
One of this year's jurors, Nancy Elsmo, head of children's services at Wisconsin's Racine Public Library, said she finds the process ''exciting . . . and scary. I'm most concerned about eliminating a book too early.'' The real reward, though, she says, is having the opportunity to become thoroughly familiar with a year's worth of new picture books.
Named for Randolph Caldecott, the noted English illustrator, the Caldecott Medal was first awarded in 1938, under the sponsorship of Frederic G. Melcher, then coeditor of Publisher's Weekly.
Illustrators and-or authors of the winning books will receive their bronze medals at the ALA convention this summer in Los Angeles.