The power of storytelling is the key both to the shape of the welcoming Weston Theatre, the main auditorium of the splendid new Unicorn, and of its opening production.

The striking building just behind City Hall, near London Bridge Station, is the first purpose-built theatre for children in the UK, and architect Keith Williams has done young audiences proud. The involvement of children from a local school who became “young consultants” and of artistic director Tony Graham and his team has resulted in an elegant, child-friendly but not childish design. As well as the Weston, there is a smaller space, the Clore Studio, an education room, cafe and all the usual facilities, with a most unusual computerised art installation in the foyer.

Children filing in to watch Tom's Midnight Garden travel back in time from a boring 1950s flat to a Victorian garden and sit on comfortable curved and raked blue bench seats from which everyone has a good view of the action, set in Tom’s bedroom, the garden, the hall of the mystical once-great house and Tom’s brother Peter’s room. Peter, confined with measles, reads letters marked BAR (burn after reading) describing Tom’s strange, time-slip adventures when the grandfather clock strikes 13.

Tom (played perhaps a little too irascibly by Rudi Dharmalingham) regularly meets daring Hatty (Debra Penny) in the garden which, by the 1950s has been replaced by houses and dustbins. Tom’s visits are nightly, but Hatty’s time runs differently and, although she is usually about his age, sometimes she is a toddler, sometimes almost grown-up. Is Tom a ghost in her world or she in his?

David Wood’s adaptation adheres faithfully to the plot of Philippa Pearce’s still-popular novel. Graham’s production (which won a TMA Award in 2001) succeeds in drawing in the young spectators, deliberately relies on the resources of theatre and requires concentration and a willing suspension of disbelief on the part of the audience. Musicians appear on stage playing clarinet and violin music (by Stephen McNeff) to suggest the magical atmosphere of the garden and Tom passes through walls by pressing apart flexible rods in Russell Craig’s adaptable design.

The magic seems to work: on my visit, when the secret of the straight-backed elderly owner of the house, Mrs Batholomew (Ellen Sheean), was revealed in the final scene there was a burst of spontaneous applause, an expression of simple pleasure, as she hugged young Tom.

- Heather Neill