It's that time of year, when the Children's Radio Theater of Washington presents its annual Henny Penny Playwriting Contest for young playwrights.

If you're between the ages of five and 17 and enjoy writing, you may enter the competition by submitting a copy of your play no later than December 15. Children from all 50 states and Canada will be allowed to enter.

Four winners will be selected by a national panel of judges in early spring 1985. The winning plays will be produced by the Theater's company of actors, musicians and sound artists and broadcast live on public radio stations nationwide from the JFK Center. The winning playwrights will spend a weekend in Washington with the Children's Radio Theater and will be able to perform in their own play.

For a complete list of rules, write to: Children's Radio Theater, 1314 14th St. NW, Washington, DC 20005 or call 202/234-4136.

All the following activities are free unless otherwise noted.

OXON HILL FARM -- Open seven days a week, 8:30 to 5, Oxon Hill Rd. (take Beltway exit 3A, turn right on Oxon Hill Rd.,). Sunday, 12:30 to 3, learn to make cornhusk dolls. 839-1177.

NATIONAL AQUARIUM -- Open seven days a week, 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. Call 377-2825.

CEDAR HILL -- Open seven days a week 9 to 4, 1411 W St., Anacostia. Cedar Hill, Frederick Douglass' home, recalls his work with films, interpretive exhibits and a house tour. 426-5961 or 426-5962 or 426- 5963.

FRYING PAN FARM PARK -- Open seven days a week, 10 to 6, 2709 West Ox Rd., Herndon. A working model farm with animals such as goats, rabbits and peacocks. 437-9101.

USS BARRY -- Seven days a week, 10 to 5, including holidays, Washington Navy Yard. The Navy destroyer's weather deck is open for public viewing and self-guided tours. 433-3377.

"A CELEBRATION OF FLIGHT" -- Seven days a week, every forty minutes starting at 10:50, Albert Einstein Sky Theater, National Air and Space Museum. The successes and failures of flight in the last two hundred years. Adults $1.50; children 75 cents. 357-2700.

CHILDREN'S MUSEUM -- Monday through Saturday, 10 to 4, Sunday, 1 to 4, Rose Hill Children's Museum north of Frederick, Maryland. Take U.S. 15 north to Exit 8 to Motter Ave.; follow Motter to left onto 14th St. and left onto North Market St., 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.

DISCOVERY THEATER -- Tuesday through Friday, 10 and 11:30; Saturday, 1 and 3: Arts and Industries Building, 900 Jefferson Dr. SW. The Sierra Puppet Theater presents "The Adventures of Monkey King," an adventure-packed tale of mythological lore performed in traditional Chinese shadow puppetry. $2.50, discounts for families and groups. 357-1500.

CAPITAL CHILDREN'S MUSEUM -- Tuesday through Saturday, 10 to 4; Sunday 1 to 5, 800 Third St. NE. Saturday and Sunday, museum hours, use leaves, acorns, twigs and other objects to make a nature print: Saturday, 1 and 2:30; Sunday, 2:30, the fairy-tale, "The Magic Mirror" will be performed with hand-and- rod puppets. $1.50 for show admission. Museum admission, $2. 543-8600.

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

NATIONAL COLONIAL FARM -- Tuesday through Sunday, 10 to 5, 3400 Bryan Point Rd., Accokeek. Sunday, 1 to 4, a demonstration of the colonial method of soap making using wood ashes and animal fats. Adults, $1; children under 12 free. 301/283-2113.

CLOISTERS CHILDREN'S MUSEUM -- Wednesday through Friday, 10 to 4:30; Saturday, 12 to 4, 10440 Falls Rd., Brooklandville, Maryland. Saturday, 2, Baltimore's chapter of Clowns of America will clown around with magic, as well as stunts and comical skits. An exhibit of Marjorie Coffey's fabric-covered free-form sculptures and hanging banners will be on display through December 15. $2 donation requested from adults. 301/823-2551.

RAILROAD STATION MUSEUM -- Wednesday through Saturday, 11 to 4; Sunday, noon to 5, Main St. and Maryland Ave., Ellicott City. The renovated station has a diorama of the trip of Tom Thumb (an early 19th-century train) from Baltimore to Ellicott City. There are also sight and sound shows, model train exhibits and tours of the museum. Adults $1.25; under 13, 75 cents. 301/461-1944.

CLAUDE MOORE COLONIAL FARM -- Wednesday through Sunday, 10 to 4:30, 6310 Old Georgetown Pike, McLean. Observe a farmer's family performing 18th-century agricultural and domestic activities. Adults $1; children under 12, 50 cents. 703/442-7557.

ANACOSTIA NEIGHBORHOOD MUSEUM -- Friday 10 to 6; Saturday and Sunday, 1 to 6, 2405 Martin Luther King Jr. Ave. SE. Afro-American history lectures to ages four through eight as well as group tours and teaching services. 287-3369.

TWILIGHT WORLD -- Friday, 6:30 to 8:30, Huntley Meadows Park, Harrison Lane and Lockheed Blvd., Hybla Valley. Wear waterproof shoes and walk through the woods with a naturalist and discover what happens in the woods after dark. Cancelled in case of rain. 768-2525.

CHILDREN'S THEATER -- Friday, 7:30; Saturday, 1 and 3:30; Sunday, 3, Heritage United Presbyterian Church, 8503 Fort Hunt Rd., Alexandria. The Mount Vernon Community Children's Theater presents an all-children's cast of "Snow White and the Seven Dwarfs." Adults $4, children $2. Reservations, 768-7689 or 768-7841.

CHILDREN'S RADIO THEATER -- Saturday, 8 a.m., WPFW 89.3 FM. Afraid of his own shadow, "The Emerald Prince" needs help from a dwarf and a dragon. Listeners can participate in a call-in following each program by calling 783- 3104.

BIRDWALKS ON ROOSEVELT ISLAND -- Saturday and Sunday, 8:30, Roosevelt Island Parking Lot, by the bridge, George Washington Parkway (Northbound side only). Bring binoculars, wear subdued clothing and learn what birds look like and how they sound. Reservations required. 285-2598.

SATURDAY MORNING AT THE NATIONAL -- Saturday, 9:30 and 11, Helen Hayes Gallery, National Theater. Singer Farry Louis Polisar sings such original songs as "Never Cook Your Sister in a Frying Pan." Reservations recommended. 783-3370.

MASK WORKSHOP -- Saturday, 10, National Museum of African Art, 318 A St. NE. Family groups may look at African masks to discover art elements and create their own masks. Registration required. 287-3490, ext. 32.

CHESHIRE CAT -- Saturday, 11, 5512 Connecticut Ave. NW. Sing along with folksinger Cathy Fink as she performs songs from her new album "Grandma Slid Down the Mountain." 244-3956.

NATURE WALK -- Saturday, 11 to noon, Hidden Pond Nature Center, 8510 Greeley Blvd., Springfield. Explore the autumn colors and search for bird nests on a walk with a naturalist. 451-9588.

BREAK DANCE CONTEST -- Saturday, noon to 4, George Marshall High School, 7731 Leesburg Pike, near Tyson's Corner Mall. Farirfax County residents under 18 years of age may enter this contest. Sponsored by the Fairfax-Falls Church Community Services Board. 321-8294 or 281-6420.

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

STORYTELLING -- Saturday, noon, Museum of African Art, 318 A St. NE. African folktales will be the focus of films and stories told by Museum docents. 287- 3490.

CHILDREN'S PLANETARIUM -- Saturday and Sunday, 1, Rock Creek Nature Center, 5200 Glover Rd. NW. Minimum age four, ages four to seven must be accompanied by an adult. 426- 6829.

NATURE CRAFTS -- Saturday, 1 to 3, Potomac Overlook Nature Center, Marcey Rd., north Arlington. Children ages seven through 12 can make crafts from nature. Reservations required. $2. 528-5406.

SPINNING DEMONSTRATION -- Sunday, 1 to 4, Sully, Sully Rd., Chantilly. A demonstration on the 18th century craft of spinning wool and flax. Adults $2, children $1. 437-1794.

LITTLE RED RIDING HOOD -- Sunday, 2:30, Smith Theater, Howard Community College, Little Patuxent Parkway. The Candlelight Concert Society will present this children's classic as an opera in one act. $2.50. 730-3125.