The young woman playing Sir Gawain's horse ducks her head and bends her wrists to simulate an equine motion. Sir Gawain himself, an amiable fellow with a likeable face, speaks eloquently - in sign language. He has, as in all good fairytales, a quest to make and must endure three days and nights of temptation in the cold north country to save his own life.

What sort of children's fare is this, with nary a Cookie Monster in sight?

"The Imagination Celebration," a festival of arts for children this week at the Kennedy Center provides many kinds of entertainment for young audiences, and, for the most part, without the gags and cliches of children's television. The shock may prove too, much for some members of the younger set attending their first performance of anything not played out on a screen, but the quality of the productions, commissioned for the occasion by the Center with the Alliance for Arts Education, and presented free for the public this weekend in the Eisenhower Theater, is quite exceptional boding well for further commissions for children's theater in the future.

"Sir Gawain and the Green Knight" is Jamaican poet-playwright Dennis Scott's adaptation of the 14th century poem into graceful, literate but unobtrusive verse. A spare, fanciful set adn well-choreographed, expressionistic staging underline fine performances by members of the Little Theater of the Deaf. But the most remarkable thing about this production is the manner in which visual and verbal communication are interviewed.

If a medieval tale requires a special leap of the imagination into a far-removed time and place, where language - whether spoken or signed - is more metaphorical, more beautiful and complex than ordinary language, "Jim Thorp - All American" requires on the part of a young audience a willing suspension of disbelief.

An odd mixture of tones - comic and pathetic - and an odder mixture of messages, this pay by Saul Levitt "(The Andersonville Trial," "The Trial of the Cantonsville Nine") purports to be the story of Jim Thorpe, the Indian athlete whose 1912 Olympic medals were taken away when it was learned he has played pro baseball in North Carolina. There is plenty of broad humor and slapstick of the well-placed-bite and seat-of-the-pants variety, which children seem to love, and an engaging disregard for authority figures, distinguished from the good-guy Indian characters mainly by their long gray military coats.

But the stereotyped portrayal of carefree and woodsy "honest Injun" life is old enough to have appealed to James Fenimore Cooper (the Noble Savage" man) himself, and the stagey use of a sententious medicine man chorus jostles the straightforward comedy of Ji's sporting exploits.

Patricia Birch, choreographer for "Hot Grog," "Grease," and "A Little Night Music," and Doris Chase, whose scupture has appeared in numerous one-woman shows, are talented artists, and "Light Sings" is a slick, well-designed show. But there is something missing in the execution, and it seems less a production for children than a production which imagines itself to be what children, especially inner-city children, are supposed to like.The dancing is, on the whole, rather graceless, and the giant-screen-film, lighting and musical effects, while lively, are reminiscent of television and provoked few oohs or ahhs from the audience.

Interestingly, children responded to John Kauffman and Wayne Johnson's "The Indian Experience" - which was previewed in the Eisenhower this week, but it will be performed in the Chautauqual Tent on Saturday morning - at least as eagerly as they did to the larger, grander productions. Kauffman, who has a kind of Fonzie-in-desert boots appeal, told stories and explained various aspects of reservation life.

Free tickets are still available at the Eisenhower box office for "Sir Gawain and the Green Knight" on Saturday afternoon at 1, and for "Jim Thorpe, All-American" on Friday night at 7, Sunday morning at 11, and Sunday afternoon at 2. "Light Sings" will be performed on Friday night with "Jim Thorpe" and "The indian Experience." Any seats which are not occupied five minutes before performance time will be available on a first-come, first-served basis.