Q: Can you please comment about movies (such as "E.T.," "Heidi's Song," etc.) for young children? My son is 3 1/2 and I feel he is too young for such things. Are there any movies for the under-5 age group?

A: Any parent who's taken a 3-year-old (or 5-year-old) to "E.T." probably knows your concern is well-founded. At first this child is devastated to think E.T. has died, and then he is inconsolable when E.T. leaves Earth.

Most preschoolers are too young to accept most commercial movies, even those made expressly for children. "Snow White" is enough to make them boycott apples forever, and if that doesn't, the apple-throwing tree in "The Wizard of Oz" should do the job.

This is because a 3-year-old doesn't think like an adult, or even like a third grader. A scary book may not upset a young child because he only takes in as much as he can handle, but a movie supplies all the words, music and pictures. This can be haunting, for all the encoding and decoding is done for him. There's no room for his imagination to hide.

Regular movies also are usually too long to keep a young child contented.

Instead, you might begin by looking at the film resources at your local library. The films, usually about 5 to 8 minutes long, are shown in the District and at the county branches. They also can be borrowed to show at home if you have the equipment.

Story hours, scheduled at least a month in advance, generally feature two to three films with breaks in between.

Parents might read the new book, What to Do When the Lights Go On by Maureen Gaffney (Oryx Press, $22.50), director of the Children's Media Center, New York. She also wrote an earlier classic in the field, More Films Kids Like, which was preceded by Films Kids Like by Susan Rice (both $10 and published by the American Library Association). All three books recommend movies of all lengths and suggest activities and dramatics that reinforce the stories.

The A.L.A. also has just published its "Notable Film List"--as well as its lists of notable filmstrips, recordings and books--all compiled by the American Library Services to Children. For a free copy of each list, send a stamped, self-addressed envelope to the ALSC at the A.L.A. headquarters, 50 E. Huron St., Chicago 60611.

Many of the best storybooks have been turned into movies, using the illustrations from books like Rosie's Walk by Pat Hutchins, Where the Wild Things Are by Maurice Sendak, the Madeline stories by Ludwig Bemelman and Curious George by H.A. Rey. There also are some imported dandies, which are charming and wordless: the "Mole" films from Czechoslovakia; "The Sand Castle" from Canada and two super French numbers, "The Merry-Go-Round Horse" and "The Golden Fish." And then there is "A Boy, a Dog and a Frog," which is live action, and "Charlie Needs a Cloak," a real winner.

Some other films, like "The Red Balloon," may be designed for somewhat older children, and "The Red Ball Express," may be too, but it's a great deal of fun. "Homer Price and the Donut Machine" delights a first grader and so does "Paddle to the Sea," the story of a toy Indian in a toy canoe going down a real river.

These and others--most of them classics--also are shown at many of the Smithsonian museums, with the schedule announced in advance.

As the content gets older, so does the length of the movie. "When Knights Were Bold," a British movie, is 17 minutes long and good for 8-year-olds and up. "The Making of Star Wars" is about 55 minutes and enthralls a 9-year-old, and those who are 10 and older will be as enchanted by "Peter Pan," "The Rescuers" and "The Secret Garden"--all feature-length movies--as you were.

The libraries also have a couple of innovations you may not know about. One is the Penny Theatre, the little showcase once used in the court of Louis XIV and later sold in England (for a penny), and treasured by such youngsters as Robert Louis Stevenson.

There are dozens of these Penny Theatres--or primitive puppet shows--traveling around the area's library systems. Almost always made by parents, they usually include their own overhead lights, footlights, sets and a curtain. Librarians have used volunteer time to create the characters, often by cutting them from worn-out storybooks and gluing them to wires. They then write a script and cut a tape, with music and sound effects. (NASA once provided the sound of a heavy body splashing into the water.)

Many branch libraries, both in the city and the suburbs, also have bedtime stories. These weekly after-supper programs are a boon to working parents, who bundle up their children for some stories and perhaps a short movie. If it's scheduled late enough, the children go in their Dr. Dentons: a pretty picture.

And when you've tried all these movie substitutes, you may still find something missing: You want to take your little boy to the real movies anyway. Do. For all the drawbacks, it's a kick to take even a young child once or twice or three times a year. Movies are part of the culture, part of the sharing in a family and part of the excitement of growing up.

You might avoid the ones "made for children" and look instead to the film revivals at the Circle, the Biograph and the American Film Institute. You'll find Danny Kaye and Charlie Chaplin; Harold Lloyd and the Marx Brothers, "Fantasia" and "The Yellow Submarine." Some are endearing, some are silly, some are as ethereal as summer clouds, but so long as you're ready to leave if he gets bored, scared or restless, you'll be giving your child a great time. He's been invited into the captivating world of popcorn and dark spaces and wonder all around.