The child stars of Michael Winterbottom's Everyday

It is late afternoon in a red-brick house in north Norfolk, and the four Kirk children are squished on to the sofa, still in their school uniforms, discussing the art of fake fighting. "I fake-punched: I stopped about that far away," explains Shaun, his eyes broad and blue, hands held apart to show the proximity of his punch. "We had to pretend to hit 'em, coz we weren't actually allowed to actual hit 'em, because if we did we'd get into more trouble."

Stephanie, Robert, Shaun and Katrina Kirk are the four stars of Michael Winterbottom's Everyday, in competition at the London film festival this week. Shot intermittently over five years, it features John Simm and Shirley Henderson as a couple coping with his imprisonment for theft. As Simm sees out his jail term, we watch Henderson struggling to care for their children: the challenge of making ends meet, the difficulty of providing enough discipline and love, the loneliness of waiting.

Key to the film's success is its steady rhythm of repetitions: the family home all dolled up each Christmas, the classrooms, the workplaces, the procession of buses and trains in the long journey to visit Simm, the visiting rooms with their plastic chairs and Formica tables. These airless scenes are interspersed with glorious shots of the Norfolk countryside wheeling through the seasons: rippling wheat fields and broad stretches of sand, poppies, hedgerows. Behind them swells a score by Michael Nyman, a frequent Winterbottom collaborator.

The Kirks were first approached in 2007. "Michael had toured around looking for one small boy around Shaun's age," explains Sarah, the children's mother. "They wanted someone who looked like they could be John's son, and they'd done eight or nine primary schools. We had a letter home from school to say they were doing a short film. Then the school rang up and said they wanted to film Shaun playing in his home environment. They didn't realise until then that I had three other children."

Six weeks later, the film-makers were in touch to say they wanted to cast all four children. ("It was Shaun we cast first," Winterbottom tells me later. "He has this amazing face: there's something natural, but also something so expressive. But of course all the children were great.") Their parents were hesitant. "We just thought they were weirdos," their father, Colin, says bluntly. "When we first heard about the film I said, 'Does that sound a bit weird or what?' We had to check up on the internet, make sure they were genuine."

Their fears were allayed when Winterbottom visited to explain what the project would entail, and what he hoped it would produce. "And they seemed genuine," says Colin. "Just nice people. So I said to the kids, 'Do you want to make a film?' They said 'Dunno.' So I explained what it was about and they said, 'Yeah, all right.'"

Filming began a few months later, with the family home doubling as a location. Winterbottom issued the parents with strict instructions not to redecorate for five years, though they rebelled after two ("because it was falling to bits," says Robert). Sometimes a shoot would last a week, sometimes three or four days – but they started at 6am and ran late into the evening. Sarah says the children had breaks and plenty of sweets to see them through, but often the camera shows them looking exhausted and tearful.