DEEP IN THE crypt-level recesses of the Washington National Cathedral is a well-kept secret: a hands-on museum where kids can discover for themselves what life was like during the Middle Ages.

The Cathedral Medieval Workshop is a place where visitors can create a stained-glass window, lay a flagstone patio, design gargoyles or grotesques, carve limestone knights, support flying buttresses and otherwise imitate life in the Middle Ages. Originally limited to school-age field trips midweek, this hands-on activity center is now open for families every Saturday.

The center is self-directed. Instructions and well-illustrated diagrams are carefully laid out at each of seven tables to guide parents and children alike on each project. One staff aide and two volunteers are on duty to answer questions, give added history and otherwise assist visitors.

On a recent Saturday, a 10-year-old intently modeled out of clay his own version of the 103 grotesques and gargoyles found on the cathedral. From the printed material on the table, he learned that these images of beasts or ugly creatures were chosen by people living in the Middle Ages to keep evil spirits away from the church. They had a functional purpose as well: Gargoyles are actually water spouts to carry rainwater away from the roof; grotesques, with no openings, allow water to run over the top and away from the outside walls of the cathedrals, preventing water damage to the limestone.

Towering above him were winning creations from a National Geographic World Magazine "Draw a Grotesque" contest, including grotesques shaped like a raccoon, Darth Vader, "Bertha's Braces" (my favorite, a child's view of beastly orthodontia) and the winner, "Sagacious Grotesque," a beast holding an umbrella to protect itself and the cathedral from the dreaded rain. Finished copies of these winners were actually carved by the cathedral's mason and mounted on the cathedral.

At a nearby table, a 6-year-old girl painstakingly copied medieval designs in needlework as a staff aide explained that needlework pew cushions were very popular 500 years ago. Across the way two sisters, 6 and 8, donned safety goggles and carver's aprons and took turns chiseling a slab of limestone in the hopes of successfully carving the knight in front of them. By working in clay and then limestone, children learn the process that goes from artist's design to finished statue.

At the next table, a father and his 9-year-old son played "The Medieval Tool Game," learning the implements necessary for the various crafts of the time: mortar maker, carpenter, stone carver, quarrier, mason, blacksmith and glassmaker. Along the way they find tongs, chisels, wedges, trowels, blow pipes, saws, axes, planes, augers, hoes, sledgehammers, crowbars, anvils and mallets.

While a 4-year-old and his mother combined skills to complete a flagstone patio, two 8-year-old girls teamed up to make a stained-glass window, using pre-cut glass pieces and easily cut, malleable lead to follow an intricate pattern designed by staff artists. (When you visit, note the stained-glass window above this project area. There's an eerie resemblance between the soldier depicted in this 15th-century French window and the 20th-century Darth Vader grotesque across the room.)

One of the most popular exhibits (and a definite hit with my two daughters) is the building project where participants don hardhats and imagine they are the building crew for the National Cathedral. Visitors piece together stones to make a Roman arch or carefully balance wooden flying buttresses to support a Gothic arch. Along the way physics lessons are intertwined with the architecture: "Try to apply pressure at the top of the Roman arch. Does it still stand?" asks the staff aide. "Try to stand the Gothic arch without using flying buttresses. Apply pressure. What happens?"

Would-be medieval builders then are instructed to look at the arches in the ceiling above: Roman or Gothic? It's this type of hands-on experimenting combined with art history that makes the shared educational experience contagious. The true test for success is noted by Gloria Day, a staff volunteer: "We've had kids as young as 4 enjoy it. I'm 47 and I'm crazy about it. What often happens is the kids are ready to go and the parents aren't." CATHEDRAL MEDIEVAL WORKSHOP --

In Washington National Cathedral, Wisconsin and Massachusetts Ave. NW. 537-2930. Open Saturdays 11 to 2 on a first-come, first-served, walk-in basis. Free. Cathedral docents suggest that before visiting the workshop, visitors tour the cathedral to see its stained-glass windows, arches, flagstone patios, gargoyles and grotesques.

Sally Galvin last wrote for Weekend about skiing with children.