Long before Mo Willems made a name for himself as an author, or Stephanie D’Abruzzo won a Tony nomination for playing Lucy the Slut in Avenue Q, the two were colleagues on Sesame Street. And though they also worked together in 2000 on Sheep in the Big City, on the Cartoon Network, their career paths had diverged a bit when Mo and his wife, Cheryl, became parents for the first time, to a baby girl named Trixie.

D’Abruzzo did send a baby gift. “It wasn’t a bunny—it was a Gund lamb named Fluffles, one of those floppy, adorable, super, super, soft stuffed animals,” D’Abruzzo recalled. A few years later, she saw Willems’ book, Knuffle Bunny, in a bookstore, and heard it had won a Caldecott Honor. “It crossed my mind that maybe Fluffles was Knuffle Bunny, but it wasn’t corroborated until recently.”

Willems said when the idea of turning Knuffle Bunny into a musical first arose, he thought of only one person to play Trixie: D’Abruzzo. “I love the weirdness of it, that she would have given us Knuffle Bunny and now she’s playing Trixie on stage,” he said. “I really embrace how odd and unlikely it is that this has happened.”

Knuffle Bunny: A Cautionary Musical, written by Willems with music by Michael Silversher, had its world premiere at Washington’s Kennedy Center on May 8. The sold-out run continues through May; a national tour opens in October.

To expand the 32-page picture book into an hour-long family musical, several scenes were added to the original text, and the cast expanded to five, including two Bunraku puppeteers who help Trixie perform a moving pas de deux with a much-larger-than-life-size Knuffle Bunny in a dream sequence. The puppeteers also play the combatants in a black-light scene in which Trixie’s father, played by Michael John Casey, swims through the washer’s spin cycle, tangling with an oversize bra and battling (briefly—get it?) with a giant pair of boxers, as he struggles to find his daughter’s lost lovey.

The emotional high point of the show is “Aggle Flaggle Klabble,” a tender ballad about loss, sung by Trixie, which stays true to the core conceit of the book – a preverbal toddler’s maddening inability to articulate to Dad that they have left her beloved bunny at the laundromat. The song is written entirely in babble. During the May 8 premiere at the Kennedy Center, it brought down the house.

“We believe it is the first entirely gibberish song in the history of musical theater,” D’Abruzzo said. “Memorizing it was one of the hardest things I’ve ever had to do.”

There’s also—why are we not surprised?—an attempt by a famous two-footed character from another Willems title to steal the show (Do we really have to mention who it is?) Every kid in the audience giggled when he waddled onstage.

The real Trixie Willems, now “nearly nine,” proclaimed the show “Great!” Willems, too, was pleased with the experience of stretching his talents to a new medium and being given another opportunity to refine the message he hopes to convey with the Knuffle Bunny story.

“All the Knuffle Bunny books [a third, Knuffle Bunny Free, comes out in September] are less about me and my daughter than about the kind of father I’d like to be,” Willems said. “These are love letters to my child but also about parenthood and that beautiful bittersweetness you feel as your child grows up.”