Doris Salcedo at Tate Modern

When I was an art student a group of us paid a visit to the painter Patrick Heron’s house on the coast of west Cornwall. During the time he generously spent with us – we had a tour of his house, Eagles Nest, and its gardens, and he was gloriously indiscreet in his anecdotes – he told a story of the time when he was a trustee of the Tate Gallery and was accompanying the then prime minister, Margaret Thatcher, around the gallery. As they walked through the Duveen Galleries, at what is now Tate Britain, Thatcher took to encouraging them to install larger sculptures in it, gesticulating with her right arm as she marched along. Heron imitated her for us, illustrating how uncannily at home she would have looked if she had been attending the Nuremberg rallies in Nazi Germany.

Heaven knows what she would have made of Doris Salcedo’s Shibboleth in Tate Modern’s Turbine Hall (until 6 April 2008), a space that makes the Duveen gallery appear cupboard-like in comparison. Artists commissioned to make work in the hall for the Unilever Series have to take on the fact that it is a huge space, 35,000 square feet, they are dealing with. And it goes up and up.

You have to look down to see her work, though, which is, in a kind of way, something that is not there. It is the space she has created, what she has taken away rather than what she has imported, that has attracted the crowds. The long, wandering crack, more than 500 feet of it, snakes from one end of the hall’s floor to the other. It looks authentic, as if it has been created naturally, large enough in places for fingers and ankles to go into and set the minds of personal injury solicitors racing. Nobody is looking up. Thatcher would probably twist her ankle in it, except it is most likely that she has never set foot in a gallery since that meeting with Heron.

The kids come to see it with us, with some of their cousins. It’s interesting what they like and don’t like, and the Unilever Series has captured their imagination more than once. Olafur Eliasson’s Weather Project did it, as did Carsten Holler’s slides. They run along the crack, stick their arms into it, someone falls into it and then a Tate attendant closes in on us. I feel you really haven’t experienced Shibboleth unless you have stepped into it, or at least twisted your ankle. It is the act of consummation that it deserves.

It doesn’t seem as if the Tate Modern building is falling down for the simple reason that the crack is in a gallery, just as I didn’t feel as if I was in a playground when Holler’s slides were installed. And anyway, the slides in Pirate’s Playhouse in Stoke Newington are just as good, there is a shorter queue, and you are surrounded by people who are simply out to have fun rather than read anything more into it.

Whether or not the children are touched by what Salcedo says it is about – colonial exploitation, racism and the uncomfortable truths that we are forced to confront (how Thatcher would have hated it) – they play natural games about boundaries around it. How kids respond to a piece of work is often more enlightening than how the critics do.