“The Lion, the Unicorn, and Me” at the WNO features from left, Norman Garrett as the Elk, Ian McEuen as the Hippo, Lisa Williamson as the Flamingo, Soloman Howard as the Lion, Deborah Nansteel as the Cat, Patrick O’Halloran as the Stagehand, and Wei Wu as the Lizard. (Scott Suchman)

So a lion, a unicorn and a donkey walk into a bar — along with a singing flamingo, an 11-year-old boy angel and a chorus of exuberant children — and compete for the honor of carrying Mary and Joseph to Bethlehem.

Spoiler alert: The lion is a little rough around the edges, and the unicorn, being mythical, keeps disappearing — so the angel, who’s been charged with making the selection, opts to go with, well, the obvious choice. Instead of bringing in an exotic creature from distant climes, the angel turns to the regular beast of burden in those particular parts, and the humble donkey gets the job.

In short: The setup of “The Lion, the Unicorn, and Me,” the Washington National Opera’s holiday opera for 2013 — which had its world premiere Saturday at the Kennedy Center’s Terrace Theater — lacks suspense. But if the peg is slender, the work manages to sustain a lot of fresh energy and charm — from the young cast to Eric Teague’s colorful yet simple costumes — and it left Saturday night’s audience pretty much delighted.

There are certainly plenty of extra-musical reasons to want to like this opera. It’s a new American work commissioned by a large opera company. It’s written for children, and it involves plenty of children on stage. It also turns out, soberingly — as Francesca Zambello (who directed it) noted in remarks to the audience before the curtain — to be the first opera by a woman that WNO has ever presented in its 48-year history.

It’s only the second opera by this particular woman, too: Jeanine Tesori, its composer, is best known for musical theater (“Caroline, or Change,” “Thoroughly Modern Millie”) and film scores. Zambello is taking an active role in helping her test the operatic waters. Although her involvement with opera was initially sparked by an invitation to participate in the Metropolitan Opera’s commissioning program, her first two operas have been commissioned by Zambello: first the one-act “A Blizzard on Marblehead Neck” (with a libretto by Tony Kushner) for the Glimmerglass Festival, and now this opera, an adaptation of a children’s book by Jeanette Winterson, with a libretto by the poet and frequent librettist J. D. McClatchy.

Given Tesori’s résumé — she also wrote “Shrek the Musical” — it’s no surprise that her music is adroit and polished, with plenty of quotes from other genres: a jazzy number for the flamingo, and a few signs that she’s been listening to Ravel’s magnificent short fairy-tale opera “L’Enfant et les sortileges,” an eminently worthy model. Kimberly Grigsby, the conductor, was making her company debut, but she is no stranger to Tesori’s music, and she kept things moving energetically if sometimes a touch heavily.

Under Zambello, WNO has been keeping the young artists in the Domingo-Cafritz program particularly busy. Having learned new music for the three one-act operas that rolled out last month, a number of these performers were back onstage with more new music for this work. Soloman Howard is by now something of a hometown favorite, and as the Lion, he got to flex his vocal muscle and was less muted than he has sometimes sounded in past appearances. Norman Garrett seemed to have as much (if not more) fun in a panoply of smaller roles, including the Elk and (along with Howard) a butcher chopping up sheep heads in the Bethlehem market. And Wei Wu sounded vigorous as the Innkeeper and Ox (among other roles). Economically, this opera calls on its singers to take on numerous bit parts: Deborah Nansteel and Patrick O’Halloran were also pressed into service as the Cat and Joseph, respectively.

But the WNO didn’t stick only to its young artists for casting. Several other young singers made company debuts Saturday: Ian McEuen, showing a promising tenor as the Holy Man and Dog, in particular (he also played a hippo); Lisa Williamson, slightly and perhaps purposely strident as the Flamingo; and John Orduña as the solid, reliable, unglamorous Donkey. Catherine Martin brought some operatic wattage to the part of Mary. And young Henry Wager, in a wonderful, refreshingly natural performance as the boy Angel, did what young performers are supposed to do in this kind of thing, and just about stole the show.

Having set up the first act as the aforementioned competition with the animals, the opera continued with a second act that awakened comparisons with a church Christmas pageant: the journey to Bethlehem, “no room at the inn,” the birth of Jesus and the flight to Egypt. Despite some nods to cleverness, including McClatchy’s repeated use of tag lines (the lion’s roar, for instance, is supposed to terrify everyone else onstage), it seemed oddly straightforward religious content to find in an opera house. When the audience responded to the Nativity tableau with applause, I fully appreciated for the first time the fact that religious music is usually received in silence. I surmise that the creators had in mind, to some degree, the most popular holiday family opera of recent times, “Amahl and the Night Visitors,” which offers another view of the Christmas story.

I’m not sure this work will rival “Amahl” in future popularity, but it’s an engaging and worthy entrant in the long lists of children’s opera, and I hope that it does indeed see future productions.

The Lion, the Unicorn, and Me

continues next Friday and Saturday evening, and Saturday and Sunday afternoon, at the Terrace Theater. www.kennedy-center.org.