When director David Harradine started work on a new children's show about light, he did not know it would end up being about darkness. But that's a child's imagination for you. Think back to when you were three. Was there anything scarier than the moment your mum flicked that switch?

Opening in Polka's Adventure Theatre next week, Brilliant is the latest work from Fevered Sleep, the theatre company Harradine set up in 1996 to produce visual, site-specific and installation performance for audiences of all ages, particularly the young.

Each Fevered Sleep project begins with its audience, in this case a series of workshops with three- to four-year-olds in London, Bath and Liverpool, in which Harradine used playtime to explore their experiences of light.

"It became clear those experiences centred round light switches, torches, bedtime and the moon," he reports, "of going to sleep and of being unable to sleep because of fear of the dark. Children have a much stronger emotional and intellectual relationship to darkness than light; to night-time rather than daytime."

Armed with this knowledge, Harradine and his team set about creating a magical 40 minutes of theatre that begins with the simple act of turning off a light at bedtime. Stars, candles, a glistening chandelier and the moon all play their part in proceedings - and darkness, too, of course.

"There are points where it's totally dark on stage and we all presumed there would be problems," says Harradine. "In fact, when we tested it on young audiences, it was the opposite. The moments when they were most thrilled were when it was darkest. It seemed to liberate them."

"I'm 37," he continues, "and I don't know what it's like to be a three-year-old any more. So these workshops are vital. As an adult artist, I don't presume anything about what a child knows and understands and likes or dislikes."

What has impressed him most is his audience's ability to grasp complex, abstract concepts. "There's a trend in theatre that everything has be very hands-on," Harradine adds. "But this play is about light being insubstantial - you can't interact physically with it. Children absolutely get the idea it's something they can play with, without them physically having to move. And that's quite sophisticated."

Just as his pre-school audience get to play at being grown-up, Harradine is thankful his work brings out his own inner child.

"Who wouldn't be?" he says. "One of the reasons why I'm really interested in making work with, and for, children is I think children and artists share something in common. They have been given permission to play and I have been given that permission, too."

Harradine pauses. "My job is to play," he reiterates, "and really, what a brilliant job!"

Brilliant, Polka Theatre, 240, The Broadway, July 9 to August 16, £7.40/£6.50, call 020 8543 4888, visit polkatheatre.com