An understudy's toughest job must be going on for the lead with minimal notice, minimal rehearsal, and an audience packed with critics. And when the majority of those critics are the harshest imaginable - children at ages still in the single digits - it's got to be downright terrifying. Anyone could be expected to stumble at least a little under those circumstances.

That David Abeles, filling in at the last minute for Todd Buonopane, not only maintained his footing but triumphed during one of the final previews of TheatreWorks USA's Henry and Mudge is half-cause for celebration. The other half is that this bright and bouncy musical by lyricist-librettist Kait Kerrigan and composer Brian Lowdermilk, playing at the Lucille Lortel Theatre through January 20, satisfies beyond its ability to showcase an actor who goes out there a human understudy and comes back a star canine.

Based on Cynthia Rylant's series of children's books, Henry and Mudge follows the friendship between young Henry Faber and the enormous, scrappy mutt Mudge his parents buy to ease Henry's transition from city life to country life. It takes a while for Henry to learn that Mudge will never be exactly the friend his cousin Annie was in the city, but they become inseparable buddies soon enough.

As Mudge is not only charged with rambunctious energy but is bigger than the adults, the role presents some difficult casting challenges. The actor playing him not only needs stature and strength enough to literally carry the other actors on his back, but must possess enough humanity to create a dog so likeable that you can understand why a visiting Annie and Henry might fight over him, and even want him to be your best friend, too.

With bulging, expectant eyes and a Grand Canyon-sized smile that droops to his feet when disappointed, Abeles has the perfect Chuck Jones malleability of features for Mudge. He speaks and sings with color and liveliness further befitting a living version of Sucie Stevenson's affectionate illustrations for the books. (Paul C. Weimer's skewed-perspective set pays its own attractive tribute.) The more Abeles romps about the stage performing tricks or rescuing Henry when he wanders off into the woods, the more he seems to channel pure theatrical joy. This resonated especially strongly with those youthful critics: Never have I heard a TheatreWorks audience squeal with wonder and delight more than the one at the performance I attended. Abeles did exactly what Mudge must: He worked his way into your heart.

His Henry, however, could not do the same. The adult Joseph Morales is utterly unable to project the appearance of innocence and youth necessary to convince as a child. He sings well and performs gamely and athletically, but his cloying line readings scream condescending children's theatre, something avoided by Abeles and Patrick Boll and Joan Hess (nicely cast as Henry's parents). Jennifer Cody's performance as Annie makes Morales's even more discomfiting: She's likely older than he is, but looks and behaves 20 years younger, creating a bossy, brassy girl possessing no trace of womanhood, who can believably and innocently usurp Mudge's affections. Compared to Cody, Morales acts like a college sophomore who slept through a final exam.

Though Henry and Mudge would benefit from better-matched costars, it's a cheerfully entertaining time anyway. Director Peter Flynn and choreographer Devanand Janki have just the right light touch for the material, which Kerrigan and Lowdermilk have injected with no end of playfulness. Their songs suggest a kid's birthday party by way of Stephen Sondheim, with charm songs, musical scenes, and standalone numbers that define the whole world from a child's wide-eyed point of view. The various Henry and Mudge duets come off the best, charting the growth of a vital, lasting friendship, though Annie also inspires some nice moments with her smarmy entrance with "My Party Dress" and when demonstrating her singular ability to get Mudge to "Roll Over."

The show only falters somewhat in its rather limited scope: While there's learning value in the rivalry that develops between Henry, Mudge, and Annie, this is otherwise a roundly episodic show content with examining the title pals' growing fondness for each other. There's nothing wrong with that, but other TheatreWorks musicals have reached higher.

However, any opportunity to see a Kerrigan-Lowdermilk musical is worth savoring. Perhaps the most important young writers in musical theatre today, they have a gift for connection and emotional insight not matched by many of their colleagues, and it manifests itself as completely in Henry and Mudge as in their adult shows; the depth of the relationship that develops between boy and dog in merely an hour is remarkable, and if you're moved to tears by their realizing their feelings for each other - well, you're not alone.

As for Buonopane, he was expected to return to the show by tonight. Here's to his speedy recovery, but even if you miss him, Kerrigan, Lowdermilk, and Abeles will make sure you're in good hands. Actually, make that paws.

Henry and Mudge
Through January 20
Lucille Lortel Theater, 121 Christopher Street (Between Bleecker and Bedford Streets)
Tickets online and current Performance Schedule: TicketCentral