Seesaw has a running time of 40 minutes “followed by free play time in the sandpit”. They got me. Fortunately, I think they also got the intended audience of under-4s, with a gentle, funny production that delicately guides us through the tricky task of making friends.

Very young children will sometimes watch the world with a particular wide-eyed seriousness, a reflection of the fact that so many things in it are new and strange, and require intense concentration to understand. It is a huge success of Malachy Orozco’s sound design that his oddly melancholy piano music conjures the suggestion of this solemn strangeness for the adults in the room, and enthrals us too.

As the play begins, the music follows Rebecca Omogbehin’s welly-booted footsteps around the edge of the big sandpit-stage. It’s autumn, a few red and yellow leaves are scattered over the sand, and the Girl draws a sand-picture of Hairy, her toy rabbit. Then she has to run to the toilet. The toilet is a much more important off-stage character than the children’s parents. There is a lovely moment later in the play when the Boy (Christian Roe) leaves the seesaw to dash off; his explanation, “Need the toilet,” is met with a wise, say-no-more, “Bye!” from the Girl.

So it’s a play of few words, but each time its simple props are used in new ways a new chapter of the story is told. It takes place over the course of a year, and the bold, bulky costumes convey this delightfully, as the Boy reappears with big white smears of sun cream on both his cheeks, or the Girl puts on a bright red winter coat. A major source of contention is the Boy’s carton of juice, and the rocky course of their developing friendship can be seen in the way each new box of juice is drunk – in the final scene, they sit on the seesaw and pass it between them, slurping and sharing contentedly. Nothing needs to be said.

What there is of Stewart Melton’s script is generally excellent, however. Its less successful moments arise when the language is self-consciously ‘childlike’. Roe and Omogbehin never strain to appear obviously childish, eschewing babyish voices or exaggerated movements, and this usually works very well – moments where the script uses childish grammar make their acting appear more unnatural than it has to be. But when it doesn’t try too hard, it is often very funny – when he’s being especially exasperating, the Girl tells the Boy, “You are an impossible man!” – and it can be quietly poignant. When the Boy accidentally pushes the Girl over in a tussle for the juice carton, for example, he freezes, panics, and earnestly instructs her to “Be brave, don’t cry,” before running off.

Playing on a seesaw, whether you’re using it to travel into outer space or to hop as high as bunny, requires a balancing act. So does friendship, and Seesaw’s unostentatious use of this metaphor is typical of the production, which holds some big truths within an endearingly simple narrative for little people.

Seesaw is playing at the Unicorn Theatre until 2 November. For more information and tickets, see the Unicorn Theatre website.