Although Lukas Foss's "The Jumping Frog of Calaveras County" revolves around a reptilian record-holder, no such creature (or anyone faking one) appears onstage in Opera Theatre of Northern Virginia's production. Instead, an inspired stage director has the young audience conjure up its own frog.

During a short overture, accompanying a clever mime-dance-acrobatic sequence for frog owner Smiley (tenor Wendell Creasy), director Debbie Niezgoda manages not only to introduce the frog but also to frame the action and set the exuberant mood. Later, at appropriate moments, a taped soundtrack envelops the hall with authoritative "ribbets" -- selected, as artistic director John Niles later explained, from more than two hours of canned frog sounds.

That respect for young imaginations paid off. Opera Theatre, a local specialist in children's opera, knows the danger of reiterating a point or heaping on the saccharine. That risk is built into Foss's transparently moralistic score, penned in 1950. The frog embodies the triumph of human and animal natures, paired in common purpose against greed and profiteering.

Saturday, humor triumphed over sentimentality. Performers made their characters accessible without lapsing into caricature. Although his significance as the First Miner was small, the talented tenor Douglas Walter provided many of the afternoon's vocal high points. Niles took a brisk, unfussy approach to the score's folksily romantic tunes.

The sets were simple but ingenious. Uncle Henry's is a small wood platform -- floating like a raft in the middle of the stage, or perhaps, like a lost soul floating in the morass of Gold Rush California's prospecting, gambling and violence. For the second scene, the stage splits into two pieces that are then placed at a right angle, suggesting a town square and community ideals.

The cavernous Thomas Jefferson Community Center does not fulfill the company's ideal. For children's opera, a more intimate space is needed, not just for ambiance but to ensure that words can be heard. But if crowds continue to sell out as they did Saturday, a smaller space will be difficult to negotiate.

By shaping the next generation of opera buffs, good children's opera is a preventive remedy for grown-up opera's aging subscribership. Evidently Opera Theatre is looking for a cure. Saturday, the production engaged kids not just in "Jumping Frog's" vivid story line, but in the very process that created it. After the performance,Niles fielded inquiries about the nuts and bolts of an art form heretofore unfamiliar to many present. Judging from the number of little hands in the air, the question-and-answer session threatened to outlast the 35-minute performance preceding it.