"Wickety-wack! Get in the sack!" cried the crowd at the Round House Theater. It was an audience to be reckoned with, so Mike Howell obeyed and hopped in Greta Lambert's sack.

This was the most dramatic moment in a tale about a bumpkin, a haunted house and a sack. The actors had asked the audience to yell "Wickety-wack!" for effect. The magic words thus shouted, the sack became a marvel for swallowing ghosts, wild beasts and even the spirit of death -- played as a friendly calypso singer by the broad- beamed, deep-voiced Howell.

"Performing for kids is great fun," Greta Lambert said later, after portraying the pipe-puffing hayseed. "Because when kids love something, they go wild."

Judging by their reaction, these kids loved it a lot. The show the other afternoon was "Here a Ghost, There a Ghost," a collection of eerie yarns presented through words, music and plain old scare tactics by the Round House's resident acting company.

In coming weeks, the theater in Silver Spring, run by the Montgomery County recreation department, will offer "Tales of the Gulf Stream," a program of nautical stories; "Little People's Music," for which singer Michele Valeri will perform; and "Mime Over Matter," with company members Michael Littman and Mark Jaster showing that silence is occasionally more zany than golden.

"This program is one of the things that the theater is here to do," artistic director Jeffrey Davis said of the Saturday children's shows. "For today's, we went to Jay O'Callahan, a storyteller who performed here last summer. With his advice, we collected hundreds of ghost stories, from which we chose the ones that would work on stage. We experimented with different techniques."

Most of the kids seemed to have a few techniques of their own. Before the show, amid the pandemonium of adults and their charges finding seats, a young girl amused herself by dragging her baby brother by the ankles down the aisle steps while he grabbed at the carpet and ran his fingers across the floor. Whenever they hit bottom -- she giggling, he gurgling -- she carried him back up for a repeat performance.

A pair of boys made fists and slapped their palms in tandem. A girl, rocking back and forth in her chair and whooping as if her life depended on it, kicked up her legs at the ceiling. But everyone got very quiet and still when the director doused the lights.

The actors slipped through a curtain to the stage. "It was a dark, dark night," they began their ghoulish incantation, which ended after a while in a startled shriek -- first from the actors, then the audience. Next came a story about a wicked blacksmith so ornery that even Hell won't accept him; a yarn from Dr. Seuss about ambulatory slacks; and the story about the sack, in which a natural death turns out to be a good thing after all.

"There are lots of ways you can tell a story and we've tried to use a few," Davis said. "I think this teaches kids something, but they also have a good time." CHILDREN'S SHOWS

There are three more kids' programs scheduled this season at the Round House Theater, 12210 Bushey Drive in Silver Spring, with two performances each day at 1 and 2:30; tickets $2. Call 468-4234. TALES OF THE GULF STREAM -- This Saturday. LITTLE PEOPLE'S MUSIC -- Saturday, January 29. MIME OVER MATTER --Saturday, February 5.