The most famous man in the literary world, if you are under three feet tall, has a trim beard and a lean physique and is currently sending the kindergartners of Raymond Elementary School into fits of apoplectic hysteria because he has just uttered the funniest phrase in the history of the universe, which is “bird poop.”

If you do not have a child, you have never heard of Mo Willems. If you have a child, you are either looking for an author like Mo Willems, or you have found him and he has become the author that you can stand to read at bedtime, over and over (and over) without psychotropic assistance.

“And now,” announces Willems, 43, with the booming theatrics required to hold the attention of a short audience, “I am going to teach you how to infringe on my copyright!”

Do you get it now?

Mo Willems is doo-doo funny for kids, but he’s witheringly funny, almost sad-funny, for their parents. His books are the barometers of taste for the toddler set. His works, say the people who deeply believe in him, mean something — something both very basic and very complex that has been distilled into cartoons and monosyllabic words.

“He really,” says Edith Ching, a children’s literature instructor and friend of Willems, “he really understands the human condition.”

Real life at a child’s level

“The thing about writing for kids is that there are no cultural modifiers. You can’t say that something ‘is so Walter Mondale.’ ” You can’t reference the Kardashians. The Kardashians are meaningless to kids. “So all you have to work with is love, anger, jealousy — it’s a very small palette, but the existential crises are ultimately the same.”

Willems has just finished up at Raymond and is consoling his vocal cords with a cup of tea at a Columbia Heights coffee shop. Such recuperation is necessary because his readings are really more like interactive marathons, involving drawing lessons and shout-outs. He presides over them with a mixture of Pied Piper and Major General. “Please, no video,” he had gently admonished a grown-up in the audience. “No flash.”

He’s in Washington for the closing performances of “Knuffle Bunny: A Cautionary Musical,” based on one of his best-selling books. The stage version premiered at the Kennedy Center last year, did an 80-city tour around the country and returned in December. Its best number is a sorrowful siren song composed entirely of nonsense words (“Aggle Flaggle”) sung by Trixie, a child who, throughout the “Knuffle Bunny” series, loves, loses and eventually outgrows her stuffed rabbit. The character is loosely based on Willems’s own daughter, Trixie.

The first book in the series won a Caldecott Medal; the others — like many of Willems’s works, beginning with his first children’s book, “Don’t Let the Pigeon Drive the Bus” in 2003 — rested for weeks atop bestseller lists.

The books are funny because they’re funny — there are underpants, and buffoon dads — and also because they’re tinged with the melancholia that underscores all great hilarity. Trixie does not think the loss of her rabbit is humorous. Pigeon would be, he says, “mortified” to learn that people were amused by his desire to drive a bus. “He thinks it’s, like, a WPA documentary of grief,” says Willems.

Book after book of existential crises, all underscoring what Willems sees as the great swindle of childhood: It is empirically terrible. “There is no such thing as a good childhood,” he says. There is no other time in one’s life, after all, when one must ask permission to use the bathroom and when that permission can be denied. “And you don’t know that it’s going to get better, because you’re too small. You have no context.”

Willems’s success and his career is based on this lack of context, the temporal nature of childhood. He has embraced this role so entirely that, at times, he appears unaware of his own cultural significance.

“My agent was just telling me” that some famous person had mentioned him on her blog, had said that she loved his books. Willems can’t remember who. He’s never heard of her. Some actress, maybe? Model? “Here, I’ll look her up,” he says, scrolling through the e-mails on his smartphone.

It is Heidi Klum.

Writing for wise souls

Willems grew up in New Orleans, raised by Dutch immigrant parents who, after their arrival to the States, went on to become a lawyer (mom) and a potter (dad). Inasmuch as Willems believes in the general unhappiness of childhood, it should surprise no one that he winces away from talking about his own. “There’s no way around the eccentricity of having heavily accented parents . . . in a conservative town,” he says. “But I think that it’s helped me write with great empathy.”

He left the South for New York, studying film school at Tisch and forming the improv troupe the Sterile Yak, an offshoot of which later became the State. It was adult humor, but silly. There was a character named Roget Thesaurus, who spoke in synonyms, and another series about the home life of a subway conductor, who spoke in static.

One of the freshmen he brought into the troupe was David Wain, now a well-known film director, whose anticipated Paul Rudd/Jennifer Aniston vehicle “Wanderlust” comes out this winter. “I’ve been working under his tutelage from the beginning,” Wain says. “His sense of humor was, with a capital ‘I,’ identical to what it is now. A lot of his books are really just sketches.”

After college, Willems got a research job that became a writing job at Sesame Street — producers found it easier to teach funny people how to write for kids than to teach education types how to be funny — which he parlayed into his own animated series. “Sheep in the Big City” was underrated, canceled after two seasons, and, for those who were paying attention, completely genius.

Some time after that, he was standing in front of the bathroom mirror when he noticed that he had a hair growing out of his ear. Ear hair, nature’s way of reminding people that aging happens, is what prompted Willems to get serious about finding his true passion. He wrote what ultimately became the first Pigeon book, in which Pigeon fails, again and again, to talk the reader into letting him drive the bus.

‘He’s not afraid’

“He’s not afraid to show kids failing,” says Willems’s friend Tom Warburton, a fellow animator. “He’s not afraid to show that bad things can happen and good things can come out of that. There’s something underneath everything he does.”

That something is . . . humanity, perhaps? Compassion? Psychological strife? Or maybe it’s something simpler, like Willems’s explanation of how he writes for children as though they are all wise souls.

“Adults and children,” he says, “are members of the same species.”

It’s one of those sentences that means nothing and everything, depending on how you read it.

“Knuffle Bunny: A Cautionary Musical” runs through Sunday at the Kennedy Center.