IF YOU haven't yet wended your way to the new Victorian Garden on the Mall, here's an historically authentic approach, via the newly restored Children's Room in the south tower of the Smithsonian Castle.

Designed at the turn of the century as the Smithsonian's first attempt to make its collections accessible to children, the small light-filled room is a fantasy garden. A trompe l'oeil arbor of vines covers the high ceiling, with gilt-edged painted "skylights" showing patches of blue, wisps of cloud and curious exotic birds peeking in. A stenciled border of interlaced birds and leaves marches around the lime-colored walls and is reflected in matching motifs on the mosaic floor.

As you enter the room from the main hall of the Castle, you'll see a large framed photograph of the room as it appeared in 1901, with bird cages suspended from the ceiling, a small aquarium, a stuffed beaver on top of a case of stuffed birds, and child-level cases filled with shells and butterflies.

In 1939 the walls and ceiling were painted over and the room became an office. But interest in its original design was rekindled when a leak in the roof exposed the trompe l'oeil under the peeling overlayer. Three years of historical architectural research led to three months of active restoration. Roland Cunningham, the Smithsonian's senior paintings conservator, spent much of those three months on a scaffold on his back, a` la Michelangelo, uncovering three-quarters of the original ceiling.

There are no exhibits in the room today beyond the beautifully restored architectural and decorative details -- that's real gold leaf on the shiny moldings, by the way. But they're definitely worth a look before you stroll out into the bed of roses just outside. THE CHILDREN'S ROOM --

In the Smithsonian Castle on the Mall. Open 10 to 5:30 daily, with access to the four-acre Enid A. Haupt Garden.