‘We made nearly half a million kids happy with a cardboard box,” says set designer Sam Wilde. “Even if that just lasted 15 minutes, that’s something I will always be proud of.” In the lockdown of spring 2020, Wilde and director Ian Nicholson created homemade online productions of Jon Klassen’s trilogy of subversive children’s picture books, starting with I Want My Hat Back, in which a ponderous bear searches for his missing pointy red hat. The trilogy went viral and is now transferring – with bigger puppets, new actors and a good deal more cardboard – to the Little Angel theatre in London. “The first show cost £16,” says Wilde. “And that was just postage!”

“I can’t stress how tiny everything was,” says Nicholson, who performed and directed the shows kneeling behind his kitchen table. The set of the first one just about fits into a shoebox, and was made from what Wilde had to hand. A bike shop that had opened down the road was throwing out cardboard boxes; the bear’s eyes were beads swiped from one of his daughter’s necklaces.

Wilde is Zooming from his home in Bristol while Nicholson dials in from the rehearsal room, a library in Portsmouth. Unable to meet during lockdown, the pair would plan everything online. Then Wilde would make the set and puppets and post them to Nicholson, who would put everything together and film, with his now wife, Susannah Stevenson – whom they both credit as the real hero of the project – acting as stage manager.

Each of the online shows was filmed in one dramatic take – a decision born out of necessity. “I don’t know how to edit,” Nicholson admits. The first one, eight minutes long, took six hours to film. They never expected the shows to go viral. “I was convinced the only people to watch it would be my kids,” Wilde says. The morning it went online, 1,000 viewers watched live. The total online audience for the trilogy would be equivalent to selling out the Little Angel for a year and a half.

For their hilarious mischief, lack of morals and embrace of casual brutality, Klassen’s books are popular with parents as well as kids. “I often think of it as the Psycho of children’s books,” says Wilde seriously. “It’s all shifty eyes and knowing looks. That’s quintessentially Jon Klassen. It’s all about hiding the action.” Nicholson’s raised eyebrows and sideways smirks do just as much for the story as the tortoise, snake or frog – all of whom swear they haven’t seen the bear’s hat.

During the pandemic, children from around the world made their own versions of the puppets. Some even recreated the full productions and uploaded them, giving the animals the same accents that Nicholson did. “I remember getting an email about this little boy who hadn’t wanted to go out after lockdown,” says Wilde. “His parents made him a bear, and they took the bear out around their town looking for the hat.”

For more than a decade, this bulbous bear has been central to the pair’s work and friendship. They met when Nicholson was workshopping a different performance of I Want My Hat Back. Wilde’s involvement was based – rather appropriately, for this book – on a lie. “I was working as the world’s worst director,” he confesses, “and I wasn’t making any money. A friend of ours was doing the music for this and he’d told the director that he knew an incredible designer.” Wilde had grown up crafting – his dad was a children’s therapist with a focus on art – so the actual design wasn’t a problem. He just couldn’t let on that he hadn’t ever done it in a professional capacity.

That version of the show never made it to the stage; the rights were snapped up for what became the 2015 production at the National in London. Until the pandemic, they had pretty much forgotten about it. “On the first proper day of lockdown,” Nicholson says, “Sam and I thought, ‘Shall we just make a show?’” They got in touch with Klassen on Twitter, who told them to go for it.

A child who watched the digital show when they were three would now be five: “Your life is enormously bigger,” says Nicholson, who has married and had a child since. The show has grown too – the new puppets seem ginormous. “This is fox,” Nicholson says, bringing out a beautiful, sleek puppet bigger than his head. “He was an inch and a half for the digital show.” Bear, who was five inches, is now more than two feet tall.

They’ve brought in two actors – one of whom will job-share with Nicholson so he can manage childcare – and the performers get to stand, rather than kneel. “Hopefully it’ll still have that same feeling of the video,” Nicholson says, “of being handmade and something you could do at home.” They’re going to sneak in little surprises and Wilde will give online workshops on how to make a variety of hats, in the hope audiences might wear them to the performances. It would be a brave child who put on a pointy red one. I hear a bear is on the lookout for a thief.

• I Want My Hat Back Trilogy is at the Little Angel theatre, London, 21 May-31 July.