THE CHILDREN gather early. They wait on the edge of the wooden stage or mill about the dance floor. Those who are seated kick at the legs of their chairs expectantly. Some of the children, like 4-year-old Benjamin Roth, bring their own toy guitars.

Many are regulars, returning weekly to Jammin' Java, a small coffeehouse and music venue in Vienna better known for catering to an audience with hip musical tastes than to those wearing Huggies. Yet for the past year and a half, since the club started running free children's concerts in the morning, minivans dominate the parking lot, and preschoolers, toddlers and babies rule the day.

The initial idea to host children's concerts can be traced to owner Daniel Brindley, who said he thought daytime concerts might increase awareness about the club's evening shows. "It's a way to tell people about everything else we do here," he says.

The big draw is Mr. Knick Knack, an energetic troubadour who performs at Jammin' Java on Thursdays as part of the lineup of children's concerts, which includes Banjo Man, Miss Belle and a trio known as Rocknocerus. Known to many parents and caregivers in Northern Virginia from his gigs at bookstores, restaurants, birthday parties and malls, Mr. Knick Knack is in his element at Jammin' Java, a place with a real stage and professional sound and lighting systems.

"Most musicians along the Eastern Seaboard know about Jammin' Java," says Steve Rossi, the Northern Virginia native behind the Knick Knack name. "It's so great to be able to play here."

No gimmicks. No costumes. With just a silly name and a genuine desire to perform, Mr. Knick Knack takes the stage with his guitar, and the kids go wild. There is screaming and yelling and cheering. They love Mr. Knick Knack. And, as it seems to several mothers sitting in the audience, he loves them, too.

"He's so earnest," says Kim Roth of Vienna, mother of 4-year-old twins Benjamin and Eliza. "You can tell he really enjoys what he does."

Roth attends the concert with her neighbors, Ann Rosenbloom, who also has a set of preschool-age twins, and Anne Roberts, who has a preschooler, a toddler and a 2-month-old baby in tow. Among them there are three adults and seven children younger than 5.

Benjamin Roth takes the stage with his tiny toy guitar, bobbing his head to the music, while Eliza dances below.

The children make good use of the club's dance floor -- shaking, grooving, laughing, spinning and twirling. Parents, for the most part, can be found comfortably seated nearby at the tables in an area that's more nosh pit than mosh pit.

Roth and her friends drink coffee and enjoy the air conditioning and one another's company, while Mr. Knick Knack works through a repertoire of original songs and old standards -- including a spirited "Shake Your Sillies Out" and an "Old MacDonald" rendition with enough animal noises to keep any child happy.

"Believe me," says Roth, "it's great to be able to be out with friends, to have the kids enjoy the music while the moms have a chance to have a cup of coffee and catch up. It's a rare thing."

For its part, Jammin' Java has made a few changes to accommodate the younger crowd, including a menu that includes standard kids' fare: juice boxes, fruit, bagels, peanut butter and jelly sandwiches and Fig Newtons. The club is always smoke-free. The performance space is well lit and offers unobstructed views of the dance floor, making it a good design for its daytime habitues. A few mothers noted that a diaper changing station in the bathroom would be a welcome addition.

A Knick Knack show has been added on Monday afternoons at 4:15 -- a quiet time, thought Rossi, who originally planned to use the slot to try out new material.

"It was such an odd, off hour, I didn't expect anyone to show up -- it's the witching hour, after all," says Rossi, referring to the time of day between naptime and dinner when children are prone to meltdown and parents are frazzled or worn down.

And yet the word of his appearance spread through the parenting grapevine, and more and more families started showing up.

On a recent Monday afternoon there were 30 children catching his show. Rossi's original songs have a definite rock edge and reflect his Knick Knackian worldview, with themes that lean heavily toward family, love and children. He says his signature song, "My Heart Is Like a Family," resonates with his audiences because the message -- about relating to others as family -- is "so true and beneficial."

"Parents come back all the time because they enjoy it -- yes, there's a message, but there's also a serious rock 'n' roll song they can relate to."

Rossi closes the show with "The Alphabet Song," a rousing singalong that incorporates "Twinkle, Twinkle Little Star." His shirt and hair are soaked with sweat after the 45-minute set. The children clamor for more, and he does not disappoint. He says he likes to send them home happy. Springsteen's got nothing on Knick Knack in the encore department, as Rossi keeps belting them out, even after the mothers have begun to pack up the diaper bags and drag out the strollers. Finally, the children are cajoled or pulled off the dance floor by parents, the spell broken, with Mr. Knick Knack's closing lines -- "You guys are the show!" -- still ringing in their ears.

JAMMIN' JAVA -- 227 Maple Ave. East, Vienna. Jammin' Java offers free children's concerts weekdays at 10:30. Mr. Knick Knack performs Thursdays at 10:30 and Mondays at 4:15. Call 703-255-1566 or visit www.jamminjava.com.

Steve Rossi, who performs under the name Mr. Knick Knack, plays guitar and sings to children, parents and grandparents at Jammin' Java. Sofia Bowater, 2, and Megan Fitzpatrick, 31/2, are spellbound.Laura Martin and son Benjamin, 4, are captivated by the show.