Skellig, Young Vic, London

His breath stinks. He is riddled with arthritis. He gobbles spiders and raw mice and leaves droppings like those of owls. Two suspicious humps bulge under the shoulders of his coat. His hygiene and dress sense would drive the What Not to Wear pair into early retirement. So, he's not the standard idea of an angel. But then, is "angel" an accurate description of this baffling creature? It's a question that is left tantalisingly open in David Almond's award-winning story Skellig, and in this engaging but overly sentimental stage recreation, directed by Trevor Nunn for the Young Vic's celebrated Christmas slot.

Kevin Wathen is just right in the role of Michael, the Geordie lad who discovers Skellig in the dilapidated garage of the new family home. His dumpy, likeable ordinariness and his air of flustered preoccupation would make anyone identify with the plight of this kid who is having to cope with the related pressures of moving house and of being sidelined by a premature and dangerously ill baby sister. Resembling the ashen ghost of a dosser, David Threlfall is excellent casting, too, as the creature, deflecting the boy's inquisitiveness with croaks of curmudgeonly Northern sarcasm.

The piece is about the power of belief. There's a mystical link between the baby fighting for her life and the terminally disconsolate Skellig, to whom Michael gives first the means (the left-overs from Chinese takeaways and bottles of brown ale) and then the will to live. Initially dismissing babies as "spittle, muck, spew and tears", Skellig is gradually drawn into taking a protective interest in the tiny child in an incubator. Metaphorically an angel who hasn't quite yet made it to earth, it is she who confers strength, finally, on the stranger.

Though my children hotly dispute this, I think Almond's fable would be more effective without Mina, the home-taught, William Blake-quoting girl from down the road. The fact that Michael allows her to join him in the adventure with Skellig gives the creature too much external corroboration. It would be a greater test of Michael's faith if he had to believe in Skellig alone, besides leaving it mysteriously moot as to whether this being is principally a projection of his own needs. And it doesn't help that, in this play, Akiya Henry has been encouraged to perform Mina as a hectoring, hyperactive know-all.

The ensemble work is attractive, with the company narrating, performing multiple roles and providing a surround that binds the young audience into the story. The evening leaps spiritedly between the mundane and the magical. Too often, though, the show succumbs to the values of a mushy musical. The songs bludgeon you with sentiment, and glutinous music mars the crucial section when Michael, Mina and their feathered friend levitate in a dancing ring of mutual belief. Young Vic Christmas shows tend to be spikier and more inventive. By those standards, Skellig is a bit of a disappointment.

To 24 January (020-7928 6363)