The world premiere of a new opera for young people composed by Gian Carlo Menotti highlights the Kennedy Center's sixth annual "Imagination Celebration," the national children's arts festival that opens Sunday.

There are four scheduled performances of Menotti's whimsical "A Bride From Pluto," a one-act "space fantasy," commissioned by the Kennedy Center, including the invitational April 14 premiere in the Terrace Theater, and performances April 15, 16 and 17. On Monday, Canada's Theatre Sans Fil (The No-Strings Puppet Theater) will present the first of four performances of "The Hobbit," adapted from J.R.R. Tolkien's novel. "The Hobbit" also plays Wednesday and twice on Thursday. Tickets for these events are on sale at the Concert Hall Box Office (857-0900).

Two free productions are scheduled in the Theater Lab. The Michigan Opera Theater presents "Alice in Wonderland," a one-act opera based on Lewis Carroll's classic fable, Wednesday, Thursday and April 10, and Metro Theater Circus of St. Louis offers "Do You Love Me Still? (Or Do You Love Me Moving?)," a dance-theater piece by artist/choreographer Remy Charlip, April 11 and 13. Free tickets to these events will be distributed at noon this Sunday in the Grand Foyer.

The Kennedy Center will also sponsor Imagination Celebrations in Tucson; Milwaukee; Marin County, Calif., and Seattle. For more information call 254-7190.