Cardboard and glue can be the stuff houses are made of. Saturday, at the Athenaeum Gallery in Old Town Alexandria, children five to 12 years old can see an exhibit of scaled-down warehouse buildings made of folded paper and then build a cardboard house, town or castle on an even smaller scale. Some of the children's works will be displayed.

Admission to the following activities is free unless otherwise noted.

NATIONAL ZOO -- Open daily; gates open from 8 to 6; animal buildings, 9 to 4:30. Elephant training demonstrations, 2; Sea lion training demonstrations, 11:30. The Panda House is again open; feedings at 11 and 3. 3001 Connecticut Ave. NW (Rock Creek Park), on the Metro Red Line. Family learning centers Zoolab and Birdlab open Friday through Sunday, noon to 3; Herplab open Wednesday through Sunday, noon to 3. Special program on Sunday: a look at the diets of zoo animals with films at the Education Building, noon; tours of the animals' kitchen, 12:30; and workshops at the Mane Restaurant, 1 and 2. For the workshops, free tickets are available at the Education Building. 673-4717.

OXON HILL FARM -- Open daily, 8:30 to 5, Oxon Hill Road (take Beltway exit 3A, turn right on Oxon Hill Road). Saturday and Sunday, 1, take a tractor-drawn hayride along the river and through the fields; meet at barnyard. Warm clothing recommended; reservations requested. 839-1176.

NATIONAL AQUARIUM -- Open daily, 9 to 5, Department of Commerce, 14th and Constitution Avenue NW. The sharks are fed Monday, Wednesday and Saturday at 2; weekday features are a touch-tank, mini- theater and a thousand types of marine life. Adults $1; children 50 cents. 377-2825.

CHILDREN'S MUSEUM -- Monday through Saturday, 10 to 4, Sunday 1 to 4, Rose Hill Children's Museum, Frederick. Take U.S. 15 north to Exit 8 to Motter Avenue; follow Motter to left onto 14th Street and left onto North Market Street, follow signs to Rose Hill. Costumed tour guides conduct tours of the 1790 mansion. Hands-on exhibits include a loom, antique toys and spinning wheel. Adults $1; under 17, free. 301/694-1646.

CAPITAL CHILDREN'S MUSEUM -- Daily, 10 to 5, 800 Third St. NE. Friday, 10:15; Saturday and Sunday, 1 and 2:30, "Aladdin!" The Puppet Company performs the story of a lamp brought to life. Saturday and Sunday, 10 to 5, paint your favorite winter scenes on a huge mural; supplies provided, no additional cost. Museum ission is $2; "Aladdin!" an additional $2.50. 543-8600.

DOLL'S HOUSE & TOY MUSEUM -- Tuesday through Saturday, 10 to 5; Sunday, noon to 5, 5236 44th St. NW. Antique dollhouses, dolls, toys, games and an exhibit of antique cat games and toys. Adults $2; under 14, $1; group rates available. 244-0024.

BENEFIT CHILDREN'S SHOW -- Friday, 7:30, Saturday and Sunday, 3; Sherwood High School's Ertzman Theater, 300 Olney-Sandy Spring Rd., Sandy Spring, Md. "Many Moons," a musical by Jennifer Boice, benefits the Sherwood PTSA scholarship fund. Advance tickets: adults $3.50; children and seniors $3. Additional 50 cents at door. Call between 8 and 4 Friday. 774-9000.

CHILDREN'S RADIO THEATER -- Saturday, 8 a.m., WPFW 89.3 FM. "The Nightingale," Part Two, a story about a cruel emperor who receives a visit from the spirit of death. Listeners can participate in a call-in following each program by calling 783-3104.

THEATER WORKSHOP -- Saturdays through March 8, 10 to noon, Joyce Kilmer Intermediate School, 8100 Wolf Trap Rd., Vienna. Children 10 to 18 can learn skills in mime, storytelling, playwrighting and auditioning; $3.50 each weekly session. 691-

PRE-SCHOOL STORYTIME -- Saturday, 10:30, Children's Room, Mt. Pleasant Branch, D.C. Public Library, 16th and Lamont streets NW. Slide shows and storytelling for pre- schoolers. 727-1361.

OLD POST OFFICE -- Saturday, noon, The Pavilion, 1100 Pennsylvania Ave. NW. A Clown Company presents "Bingo," a clown who gets children to join in his act. 289-4224.

TROLLEY MUSEUM -- Saturday and Sunday, noon to 5, Northwest Branch Regional Park, Bonifant Road between Layhill Road and New Hampshire Avenue, north of Wheaton. Trolley rides: $1 adults; under 18, 75 cents; under two, free. 384-9797.

SATURDAY AT THE NATIONAL -- Saturday, 1 and 4:30, National Theater, 1321 Pennsylvania Ave. NW. A salute to Martin Luther King Jr. presents songs, dances, music and magic, featuring young actor Malcolm- Jamal Warner. Call 783-3372, 24 hours a day.

ROUND HOUSE THEATER -- Saturday, 1 and 2:30, Round House Theater, 12210 Bushey Dr., Silver Spring (north on Connecticut, left on Randolph; make second left onto Bushey), "Dragon Tales." Chris Patton performs stories from around the world with his orchestra of computers and synthesizers; for kids five to 12. $3 per person. Advance purchase recommended. 468-4234.

CHILDREN'S PLANETARIUM -- Saturday and Sunday, 1 and 4, Rock Creek Nature Center, 5200 Glover Rd. NW. First show for children four to seven (must be accompanied by an adult); second show, for ages seven and up. No latecomers are admitted. 426-6829.

NATURE TRIVIA -- Saturday and Sunday, 1 to 4, Hidden Oaks Nature Center, 4030 Hummer Rd., Annandale. Game boards and questions allow participants to test their knowledge of plants, animals, earth science, outer space, ecology and history. 941-1065.

FAMILY FILM -- Saturday, 1:30, Twinbrook Library, Veirs Mill Road and MeadowHall Drive, Rockville. "The Secret Garden," a story about an orphan girl who goes to live with her crippled uncle and her invalid cousin. 279-1980.

ADVENTURE THEATER -- Saturday and Sunday, 1:30 and 3:30, Glen Echo Park, Glen Echo. "Dick Whittington and His Cat," a story about a boy who became rich by selling his cat. $4, group rates available; for reservations call between 10 and 2. 320-5331.

MAKE A PAPER HOUSE -- Saturday, 3 to 4:30, The Athenaeum Gallery, 201 Prince St., Alexandria. $3 for non-members, $2 for members. Parental supervision not required. For reservations, call 548-0035.

SONGS FOR CHILDREN -- Saturday, 3, Our Lady of Lourdes Church, 7500 Pearl St., Bethesda. Singers Cathy Fink and Marcy Marxer perform for children of all ages. $3; advance tickets available. 652-2239.

FOLK SONGS -- Saturday, 3, Strathmore Hall Arts Center, 10701 Rockville Pk. Chart the history of folk music with Bruce Hutton and his banjo, fiddle, mandolin and jug-band instruments as children accompany him on stage. Parental supervision recommended. 530-0540.