Tom's Midnight Garden, Unicorn Theatre, London <!-- none onestar twostar fourstar fivestar -->

What happened to the magic?

The real star of the occasion is the Unicorn's handsome new purpose-built building - a £13m theatre for children that is a wonderfully welcoming marvel of airiness and light, situated close to Tower Bridge.

When local primary schoolchildren were asked to describe their ideal venue, they suggested that the floors should be made of chocolate (well, if you have to fall to your death, it might as well be on a sugar-rush high). The building may stop short of providing such a risky snack, but with its wealth of facilities - two auditoria, rehearsal and education spaces, bright roomy foyer, art installations and a café selling ethically sound, if none-too-cheap, grub - it vibrantly achieves its ambition of offering a "serious playground" to its target audiences.

The past decade has seen an exciting theatrical renaissance in this area of London, with Shakespeare's Globe and the award-winning Menier's Chocolate Factory and Phil Wilmott's outdoor summer seasons at the Scoop by City Hall.

Children's theatre is the perennial poor relation - one notes that, once again, our flagship companies, the National and the RSC, have done nothing for the under-11s this Christmas. So it is a pleasure to see, in a permanent home on a prime site, a company specifically devoted to developing work that can reach 100,000 children a year.

I wish I could be equally enthusiastic about the opening production - a revival of Tony Graham's staging of Tom's Midnight Garden, adapted by David Wood from the Philippa Pearce 1958 classic - but the piece comes across as a stopgap while the company finds its feet in the new venue.

Though this is understandable, it's still a shame that the building is launched with a venture that does little to show off its resources. For example, the 340-seat main auditorium is a lovely flexible space that allows for in-the-round productions or ones that bring the apron stage right out into the audience. So it's disappointing that the inaugural production was originally designed for a proscenium and is therefore configured in conventionally end-on fashion.

But then, while enjoyable enough, this version of Tom's Midnight Garden, is pretty low on wonder generally. This is a damaging lack, given the fabulous storyline.

When his brother falls ill with measles, Pearce's boy hero is sent to stay with his aunt and uncle in their poky flat in a large house. He feels bored and resentful until, one midnight, he hears the grandfather clock strike not 12 but a disturbingly illogical 13 and he goes down to investigate.

At that point, he discovers that he can time-travel between the 1950s and the late-Victorian era, where delightful, orphaned Hatty is cooped up in an analogous situation, living on her imperious aunt's charity and mercilessly teased by her male cousins

It's a scenario from which theatrical magic could be woven. But Graham's production remains, by and large, uninventive and earthbound. Rudi Dharmalingham 's attractive Tom penetrates these parallel worlds by prising his way through the elasticated silver rope barriers on Russell Craig's dreary set.

We are given no glimpse of the magical secret garden (the nearest we get to foliage is the aspidistra in the aunt's hallway) and the atmosphere is insufficiently haunting, despite the bitter-sweet music provided by actors doubling as instrumentalists and the chesty chiming of the mysterious clock

True, the proceedings start to twist the heart as Tom finds that time passes more quickly in this new universe and that Hatty is drifting away from him into adulthood and rival romantic attachments.

Debra Penny touchingly traces the girl's transition into estranging maturity and there is a lovely scene where she and Tom mime the skating expedition down to Ely Cathedral where she meets her future husband. But though the production held the attention of the young audience, I doubt it will have infected many of them with the theatre bug.

To 29 January (08700 534 534)