Kinney, 45, built his not-so-wimpy empire – the books, three movies, a musical headed for Broadway and an upcoming animated TV series – by mining the high jinks and awkwardness of his childhood. Greg Heffley, Kinney’s main character, does and says things a lot like he did.

With the children sitting on the gym floor where Kinney used to shoot hoops, Kinney told stories that have made their way into his books, including the time he hid in a swimming pool bathroom and wrapped himself in toilet paper because it was cold.

“Can you tell why I called the book Diary of a Wimpy Kid?” Kinney asks.

Much howling.

“Most things in the books happened in truth or in spirit, but mostly in spirit,” Kinney says later. “I try to put it all through the fiction blender and get to the essence of the thing and hopefully get a joke out of it.”

What has made Kinney so successful, say booksellers and publishing experts, is how unusual the series was when it launched in 2007. The books are Greg’s diaries. The pages are lined like a notebook with handwritten entries and cartoon doodles – the writing sets up the joke, delivered in the doodles.

To readers, especially boys, that didn’t look like any of the books at school.

“My eight-year-old son hated to read,” an Amazon customer writes in a review of the first book. “No matter how much we worked with him he just wasn’t interested.” But Kinney’s book was, the reviewer says, “the miracle we’ve been looking for.”

It wasn’t just the format that was appealing. It was the tone. Greg is bumbling. He is not particularly popular. But he’s mouthy in a mostly harmless way.

“Let me just say for the record that I think middle school is the dumbest idea ever invented,” Greg writes on the third page of the first book. “You got kids like me who haven’t hit their growth spurt mixed in with these gorillas who need to shave twice a day.”

Jacob Baxter, 10, one of the children in the Potomac Landing gym, explains his obsession with Kinney in a letter he wrote to him in September.

“I can read your books in two days,” he wrote, adding that he had finished all the books, “so I started all over. ... You are my favourite author.”

In many ways, Kinney’s success is a total accident.

He grew up reading the comics in The Washington Post every morning with his father, a retired military analyst who worked at the Pentagon. At the University of Maryland, Kinney studied computer science and drew a comic for the student newspaper called Igdoof, about an awkward freshman with three strands of hair, like Greg.

That was the life Kinney saw for himself – newspaper cartoonist. That life never materialised. Kinney took a job at a health-care company as a computer programmer, and in his spare time he began working on a graphic novel of sorts – for adults – featuring some dorky but lovable kids. He spent four years filling his journal with doodles and one-liners. He wound up with 1,300 pages.

In 2004, he began publishing the diary entries on Funbrain, a website he worked for creating games and puzzles to help children learn. Greg had a cultish following. Hoping to find a publisher, Kinney went to Comic-Con, an annual convention for comic book writers, gamers and graphic novelists.

There, he handed his book to Charles Kochman, a well-known editor of graphic novels and comics at Abrams Books.

“I just instantly loved it,” Kochman says. “There was nothing like it out there.”

He saw it as a humour book in the same vein as The Wonder Years, the hit coming-of-age TV show starring Fred Savage that ran on ABC from 1988 until 1993. But when Kochman pitched it to his colleagues, they suggested he try it as a kids’ book. Kinney was dumbfounded. A kids’ book? It never occurred to him to a) write a kids’ book or b) that what he had already written was for kids.

But looking back, Kinney realises that not writing the book for kids is actually what made it so popular with kids. He wasn’t moralising or writing down to them.

“For me, the priority was always humour,” Kinney says.

And kids could see their own lives in Greg’s.

“The kids might know they aren’t as bad as him, but even if they are, then it’s comforting,” Kochman said. “This is not a happily-ever-after kind of thing. Authenticity really matters to kids.”

All kids. Though Kinney’s early readership skewed male, Kochman says about 45 per cent now are girls. “I think girls find the books to be a good cue into what boys think,” Kochman says. “It’s funny to them. The humour is universal.”