Lookout (Brighton Dome, off-site)

Stood on top of a multi-storey car park, leaning against lichen covered concrete, you look out over the city: a jigsaw of blocks and domes; glass, brick and slate. Seagulls fly by at eye level. People mill about below. From up here, it's a jumble: miscellaneous buildings miscellaneously arranged. You wouldn't design it like this, but this is how it's come to be.

Andy Field's Lookout wonders what it might become; how it will change over 30 years, 60 years, more. From a portable speaker, a young child's voice describes the city of the future: hovercars, suction lifts, floods. They talk of Xbox2000s and iPhone50s, of eco-protestors, mass homelessness and skyscrapers that stretch into the clouds. It's two futures stitched together, hopeful and hellish at the same time. Both seem possible. Anything seems possible. The city becomes a blank canvas: blueprint paper waiting for plans.

Children's voices are potent – and not just because we so rarely come face to face with kids we don't know. They're often ignored, dismissed as immature and unworldly, but they will define the future. Today's children won't just inhabit tomorrow's cities, they'll build them. These ideas and visions, no matter how out-there, might just come to fruition. God knows what sparked them, what newspaper reports or sci-fi films they spring from, but our world is shaping theirs all of the time. Field invites them to shape ours in turn and, in these fantastical descriptions, so boundless with possibility, they issue a challenge. What might this become? What else could it be? What more?

Recording moves to reality. A small chilld appears at your side – one of eight local primary schoolkids. Ginny's 10. She has a notepad and a bright red baseball cap, and she lives over there, on the hill, in "an old Victorian miner's house," high enough to escape the worst of the water. We get three questions, then so does she. The last one threw me: at once, an enquiry and a reprimand. It puts the future squarely on your shoulders. It's not enough to stand-by.

Lookout is a warning and a challenge: a little thing that leaves big questions behind; an intimate encounter that manages to take in the entire city. It gives you a glimpse of a possible, even a probable, future – one that may or may not include you – and charges you to do something about it. Not for your sake, or for mine, but for the future itself.

Lookout runs at Mayfest from 21 to 22 May and at Cambridge Junction from 28 to 29 May.