Jeramee, Hartleby and Oooglemore (Unicorn Theatre)

"JER-A-MEE," screams Hartleby, from the shore of the sea. "JER-A-MEE. Oooglemore? JER-A-MEE!" Oooglemore joins in: "JER-A-MEE," they chorus, until a reply comes back. "Hartleby? Oooglemore? Oooglemore. Hartleby." It's Jeramee. Who else?

As scripts go, Gary Owen's is borderline Beckett – only for 3 year-olds and up. It would give Caryl Churchill a run for her money, perhaps even Peter Handke. Owen uses only three words – three names – over and over, in various combinations. It's for the director to decide what to do with them.

In Tim Crouch's production, the trio head off to the beach: a big yellow wedge of sand with the sea where the seating starts. Overhead: the sun and an inflatable seagull. Hartleby (Amalia Vitale) struts out, cool as you like, and introduces herself. Then Oooglemore (Ffion Gill). Then Jeramee (Dorian Simpson).

In essence, it's clowning: familiar seaside rituals spun into silly routines. Hartleby undresses under her towel. Oooglemore chases a rogue beach ball. Jeramee gets tangled in his deck chair. There's no reason they couldn't go on other adventures in other shows. They're a well-balanced trio, their energies well matched to their individual names: Hartleby's all flailing-limbs and fizz; Oooglemore's a sloth-like gangle in a knitted onesie, Jeramee's upright and proper. Lily Arnold's design, vibrant and sunny, sets the mood brilliantly.

It's a cute depiction of a family unit: two siblings flicking from jibes to jokes, occasionally calling dad into mediate or join in. Vitale and Gill catch just the right conspiratorial glint, as siblings capable of communicating with no more than a look.

Owen's starting point was the children's difficulties in expressing themselves. Without language, communication's about the means, not the words: the right tone of voice, the right gestures, even the right volume.

For an audience, the joy lies in limitation and invention: the possibilities wrung out of meagre resources. You can do anything with three words. It takes three words to tantrum. Three words to play. You can needle and wheedle. You can wonder and whine.

Physically, it follows suit. At first, the siblings bicker over belongings. Their beach ball goes missing, then their balloon. But happiness is an imaginary ball – splatted back and forth – or a dance routine that develops from nothing. All you need, perhaps, is your family. They can be the whole world. Indeed, even the radio report – football scores and weather reports – comes out as such.

It's missing the reckless abandon of the best clowning; the anarchy that runs riot with what's possible; the mischief. Rather like the beach holiday it presents, it's a bit British: polite, well-behaved and literal. At no point does the beach dissolve into something else. There are no flights of fancy or folly. Clowning can do that: it can follow its own logic until it ends up as lunacy. The unexpected discovery is its chief delight, and there are just too few here. Jeramee, Hartleby and Oooglemore do everything you might expect on the beach, but they never get beyond it. As such, they rarely surprise, meaning we smile more than we laugh.

Jeramee, Hartleby and Oooglemore runs at the Unicorn Theatre until 15 March.