IF YOUR underground family fun is scheduled for the week between Christmas and New Year's, leave time to come up at the Woodley Park/Zoo station, where the free children's program "Indoors at the National Zoo" will continue all week.

Monday is "Behind the Scenes Day" (from 11 to 3), when parents and kids can see what goes on from the reptile keepers' viewpoint. You'll see how cold-blooded critters like the venomous gila monster and the five-foot Asiatic cobra are cared for and fed. You'll also see detailed files that are kept on all the animals, along with antidotes attached to the cages of the venomous animals.

On Tuesday, in the Education Building auditorium, master storyteller Sharon Butler will tell animal folk tales at 11, 1 and 2:30.

Wednesday at 11, 12:30 and 2, it's time for animal cartoons -- Bugs Bunny, Porky Pig, Sylvester and Tweety Pie -- followed by the nature film "Baobab: Portrait of a Tree," about the microcosm of life around this 30-foot-in-diameter African tree. The showings are free, but tickets must be picked up at the Education building reception desk, where you can pick up tickets for this weekend's other special activities too.

Next Saturday (January 2), the zoo's Zoolab and Herplab discovery centers will extend their hours to 10 to 3. These are hands-on areas where kids can put together bone maps, play trivia games (Dinosaur Pursuits, anyone?) and try on real turtle shells. Also in Herplab, kids can recruit the help of an attendant and take a close look at such live animals as frogs, lizards, snakes and turtles that are in plexiglass display cases with mirrored bottoms. Admission to both labs is free, but again tickets are required. (At 11:30 Saturday, hike over to the Elephant House, where the keepers will be training the animals. You never know exactly what they'll be doing -- last week, the elephants enjoyed a bubblebath.)

While it's cold outside, there are plenty of animals to see indoors -- especially those in the buildings of the "Tropical Core" -- the Monkey, Great Ape, Small Mammal and Reptile Houses. Be sure to visit the magically lit Invertebrate Exhibit, where all kinds of floating creatures live, including a quartet of giant spider crabs that arrived special delivery from Japan last week. For details of all zoo matters, call 673-4717.