Gone are all those pictures, models, costumes and memorabilia from the Kennedy Center's roof terrace "America on Stage" Bicentennial exhibit. In their place, lights, seats and platforms are being fitted for the Musical Theater Lab, co-sponsored by the Center and the Stuart Ostrow Foundation.

Starting Tuesday, artistic director Edward Berkeley, 10 performers and six staff members will be in residence to rehearse a series of productions designed to be free of commercial pressures.

Though major assignments have been made, there will be auditions for Washington performers this Friday from 10 a.m. to 1 p.m. and from 2 to 5 p.m. at the Opera House rehearsal hall. Further details about audition requirements from Berkeley at 254-3490.

There will be four weeks of rehearsals followed by two weeks of performances, Tuesdays through Sundays, free to a seating capacity of 120. These productions of new works will not be looked at as potentials for future full-scale productions. The first should begin around mid-March.

Also on the roof terrace, in the Chautauqua Tent, a Saturday series of children's programs has begun with free performances at 10:30 a.m. and 1:30 p.m. Last week San Francisco's Magic Carpet Theater offered "Kid's Writes," drawn from children's writings. This Saturday folk singer Bruce Hutton will appear. On Feb. 19 the Children's Opera Theater will present "An Introduction to Opera" and on Feb. 25 will come the Blue Sky Puppet Theater in "The Eleventh Finger." Seating is on a first-come, first-served basis.

Arena Stage also bursts with activity. Besides "The Autumn Garden" and "Streamers" on its main stages, the "In the Process" series continues in the Old Vat Room with three one-act plays: "Scooping" by Richard Nelson; "Porch" by Jeffrey Sweet, and "Exhibition" by Janet Neipris. Arena's youthful arm, Living Stage, will celebrate Valentine's Day Sunday with a benefit and auction from 3:30 to 8 p.m. at St. Mark's Church, Third and A Streets SE.

This region, Number XI, of the American College Theater Festival will hold four days of seminars, workshops and performances Feb. 17 through 20 at Essex Community College, Baltimore County. On leave from "A Chorus Line," Gillian Scalici will describe its creation, Howard Bay will talk of his stage designs, Lloyd and Barbara Meeker will explain their "New York Survival Kit," tips for career-seekers, and Larry Forde will detail a director's "Why Do It?" commands. Evening performances at 8 will present Anne Arundel College in "You Know I Can't Hear You When the Water's Running" on Thursday, Howard University in "Tambourines to Glory" on Friday and the U.S. Naval Academy in Herman Wouk's "The Caine Mutiny Court Martial" on Saturday. Sunday afternoon at 2 Frostburg State College will perform "Night of the Iguana." Further information at (301) 682-6202.

Tennessee Williams' "Small Craft Warnings" is being played under Frank Akers' direction in Back Alley Theater's uptown playhouse, 1365 Kennedy St. NW. Performances are Friday's and Sunday schedule this weekend, when the Williams' play will be presented at 7 p.m. and "Design for Living" at 9 p.m. Phone reservations at 723-2040.

With "Grease" playing through Sunday night at the National, "Bully," the James Whitmore one-man Teddy Roosevelt portriat, has deferred its opening to Wednesday, but there will be previews Monday and Tuesday nights.