Three new theater pieces for children - including a multi-media romp with sculpture, dance, poetry, music and film - have been commissioned for the Kennedy Center's first national arts festival for children.

Altogether, there will be a dozen free features during the week-long festival, "The Imagination Celebration," opening April 18.

It will be a week for children at the Kennedy Center, with puppet shows, African dances and Indian songs, legends and dances performed in the Chautauqua Tent, the commissioned works in the Eisenhower Theater, and concerts and performances in the Grand Foyer and corridors.

Jean Kennedy Smith, at a press conference announcing the festival yesterday, said she hoped the commissioned works would be the first of many "to ease what many feel is a severe shortage of releveant material for children."

The commissioned works will draw upon the talents of professionals with solid credentials in dance, theater and music. For instance, Patricia Birch, who has Tony nominations for choreography for "Grease" and "Pacific Overtures," is working on the multi-media show, "light Sings."

"It will have the poetry of children from the Streets," she explained yesterday. Doris Chase sculptures become the set, providing on "urban feeling."

The other two works commissioned for the festival will be a musical play about the life of Jim Thorpe, the great Indian athlete, and a "Sir Gawain and the Green Knight," with Jamaican poet-playwright Dennis Scott interweaving contemporary themes with medieval romance.

The week-long festival will cap the first year of a weekly children's arts series that is expected to reach more than 60,000 youngsters by May 1977. The $90.000 for the festival has come in part from the George Gund Foundation. The Alliance for Arts Education worked with the Center in planning the events.