The American Library Association yesterday awarded its prestigious Newbery Medal to Beverly Cleary's "Dear Mr. Henshaw" (Morrow) as the best children's book of 1983. The Caldecott Medal for the best picture book of the year went to Alice and Martin Provensen's "The Glorious Flight: Across the Channel With Louis Ble'riot" (Viking).

Two 15-member committees, composed of librarians, critics and teachers, wrangled throughout the weekend in closed session to choose the winners. Besides the Cleary and Provensen titles, the ALA groups also selected the Newbery and Caldecott Honor books--runners-up, in effect, to these Oscars of children's literature. The four Newbery Honor books were: "Sign of the Beaver," by Elizabeth George Speare (Houghton Mifflin); "A Solitary Blue," by Cynthia Voigt, who received the Newbery last year for "Dicey's Song" (both from Atheneum); "Sugaring Time," by Kathryn Lasky (Macmillan); and "The Wish Giver," by Bill Brittain (Harper & Row). Two Caldecott Honor books were chosen: "Ten, Nine, Eight," illustrated and written by Molly Bang (Greenwillow Books) and "Little Red Riding Hood," retold and illustrated by Trina Schart Hyman (Holiday House).

In "Dear Mr. Henshaw," Beverly Cleary, beloved by children for more than 30 years for her lighthearted novels of Henry Huggins, Ramona Quimby and Ellen Tebbits, tells of a boy, Leigh Botts, and his favorite children's book author, Mr. Henshaw. When the boy is in the second grade he writes a letter, complete with misspellings, to the famous author, who surprisingly answers; by the time Leigh reaches sixth grade a real correspondence has developed. Gradually, too, Leigh's letters reveal the facts of his life: anger over his parents' divorce, sadness at seldom seeing his truck-driving father.

By contrast, Mr. Henshaw charms with his irreverence, explaining that he writes because he has "read every book in the library and because writing beats mowing the lawn or shoveling snow." Cleary shows that the epistolary novel--even if intended for 8- to 12-year-olds--is still alive.

The Caldecott winner, "The Glorious Flight," ostensibly recounts the flying adventures of the gallant Louis Ble'riot, a French barnstormer. In fact, the Provensens' artfully naive paintings, at times reminiscent of Henri Rousseau, evoke a dreamy fin-de-sie cle France of cafe' society and stiff family portraits, of waxed mustaches and hot-air balloons, of the afternoon aperitif and Gallic elegance. Theirs is the debonair, romantic France that Americans dream of visiting and never find.

Established in 1922, the Newbery Medal is named after John Newbery, an 18th-century printer and a pioneer in the publishing of books directed expressly at children. The Caldecott, first granted in 1938, recalls the beloved 19th-century illustrator Randolph Caldecott. The medals bearing their names will be presented June 24 at the ALA's annual convention in Dallas.

Besides the honor, Cleary and the Provensens will reap more tangible benefits. Unlike their grown-up counterparts, the National Book Critics Circle awards or the American Book Awards, these two children's awards sell books. Virtually every Newbery and Caldecott medalist is still in print. Librarians give out bookmarks listing the lucky titles. Teachers recommend them. Confused parents rely on the golden words "Newbery" or "Caldecott" when they buy birthday presents.

Nearly every notable children's author--Maurice Sendak, Katherine Paterson, Madeline L'Engle, Scott O'Dell, Peter Spier--has become notable largely for having received one of these honors. In fact, it's often the only way that a contemporary children's author gets taken seriously. The major exception here is Judy Blume, phenomenally successful with kids but apparently too sexually explicit and controversial for the generally conservative views of committees.

The American Library Association represents about 39,000 members who work in public, academic and private libraries. The 1983 midwinter business conference, held this week at the Sheraton Washington Hotel, offers its 4,000 registrants a full schedule of meetings on subjects ranging from computerized catalogue systems and library funding to censorship and intellectual freedom. Secretary of Education Terrel Bell and Barbara Bush, wife of the vice president, spoke yesterday in a symposium on literacy and illiteracy in America.

Besides choosing the Newbery and Caldecott, the ALA also compiles influential lists of the most notable movies, recordings and filmstrips for children and young adults.