THE VELVETEEN RABBIT is about to become real. Not the floppy-eared little stuffed bunny itself, but the story.

If you've read the book, a children's classic by Margery Williams, you know that the toy rabbit became real after the boy had loved him for a long time and really real after the fairy godmother rescued him from the trash can. But this Saturday afternoon, "The Velveteen Rabbit" comes to life on the Adventure Theater stage in Glen Echo Park, where it will run weekends through June 8.

A few days before the real thing, the cast and crew of "The Velveteen Rabbit" are practicing popping out of boxes, learning to talk through horse masks, hemming costumes and coordinating the action onstage with the lights.

"This is a 'tech' rehearsal," explains producer Bea Hessick, who's looking through an illustrated version of the story to get ideas for makeup. Volunteer carpenters are hammering and sawing to put the final touches on the set, and director Katie Svoboda is warming up the cast of 15, most of them children.

"You guys can stretch out and get ready by just shaking out," she directs.

Leslie Rosberg, as the clown, cartwheels across the stage. Carlyn Kolker stretches her jack-in-the-box arms. Becky Stark, as the top, spins around on one foot.

"This is the first big part I've ever had. I was thrilled to get it," says 10-year-old Laura Heisler, who plays the title role. The first act has just ended on a happy note, with the boy finally adopting the long-neglected rabbit as his bedtime companion, and the cast is taking a short break. "They asked us what parts we wanted to read, and I read the rabbit," says Laura. "I just like that character. He's kind of like me. I'm quiet and shy."

A fifth-grader at Carderock Springs Elementary School, Laura played a mouse in Adventure Theater's production of "Dick Whittington and His Cat" and a wood elf in "The Hobbit."

"I want to be an actress more than anything," she sighs, trying to adjust her headdress so it won't fall off.

The rabbit's best friend in the toy closet is the skin horse, a venerable old toy who was made real by the boy's uncle. Robbie Ross, who is "12 but I'll be 13 real soon," plays the horse through a mask that is "actually pretty comfortable but it suffocates you." Robbie played an elf and a grocery boy in "The Hobbit" and has taken classes at the Folger Theater and at Adventure Theater, but doesn't know if he wants to become a fulltime actor.

"I don't know how good I am, but it's fun," he says. "We've got the book at home. It's sort of sad . . . I had a teddy bear. His eyes are out and most of his hair is falling out, but I still have him . . . . "

Robby Peckerar, who plays the boy with life and death power over these and other toys, has his own stuffed rabbits at home, named Hopalong Cassidy and Hopalong Cassidy Jr.

"I like stuffed animals," says Robby, who is 11 and has played in local productions of "The Music Man," "Once Upon a Mattress" and "Peter Pan."

"I just went to all these tryouts that I read about in the newspaper," he explains. An accomplished singer, he has taken lessons at Dale Music Center.

"They taught me how to sing diaphragmatically," he says. "You push your stomach in when you breathe out."

Robby may become a professional actor but he also likes pyschology, writing stories and plays, and making money. "Right now I'm selling chocolate lollipops. I make them from molds," he says.

As for his current role, he says, "My favorite part is when the rabbit becomes real, and the worst part was just now when my bathrobe was stuck on and we had to keep ad libbing."

"Places for Act II!" yells Svoboda, and the toys scramble back into the toy closet. All except the rabbit, who waits stage left until the moment when he will become real at last. ADVENTURES IN THEATER

"The Velveteen Rabbit opens this weekend with performances Saturday and Sunday at 1:30 and 3:30. Performances continue at the same time every weekend through June 8, except that there are no performances May 31 and June 1. Adventure Theater is in Glen Echo Park in Glen Echo, Maryland. Tickets are $4. For information and reservations, call 320-5331.

Adventure Theater's next production, a Chinese fairy tale called "The Purple Fan," will run from June 21 to July 27. Auditions for "Pinocchio," which will run August 9 through September 14, will be held at the theater June 9, 11 and 12 at 7:30 p.m. For information about the theater's summer camp programs for children five and up, call 320-5331 weekdays between 10 and 2. MORE KID FARE

Other places you can see children's theater include: SMITHSONIAN'S DISCOVERY THEATER -- The current offering is "Aladdin and His Wonderful Lamp," recommended for children in grades k-4. This puppet show runs Tuesday through Friday at 10 and 11:30 and Saturdays at 1 and 3. Tickets are $3 for adults, $2.50 for children. Call 357-1500. IMAGINATION CELEBRATION -- This weekend, the Kennedy Center's Imagination Celebration continues with performances of a musical, "The Good Life," Saturday at noon and 2 (free; first-come, first-served); a puppet production of The Jungle Book, Saturday at 10 and 11 ($7.50); and a play, "Song for the Navigator," Sunday at 3 ($7.50). Call 254-3600 or 254-7168. CHILDREN'S THEATER OF ARLINGTON -- Auditions for the summer production of "Tom Sawyer" at the Gunston Arts Center, 2700 South Lang Rd., are on April 28 and 29 from 4:30 to 6:30. Twelve boys and 12 girls between the ages of nine and 14 are needed. The play will run at Lubber Run Amphitheater from June 20 to 29. For information about the play or about summer theater workshops for children, call 739-2903.