The Kid With a Bike, Jean Pierre and Luc Dardenne, 86 mins (12A)

You want a simple, moving tale, beautifully told? Bring on the Belgians

Simplicity. It's the hardest thing to achieve in cinema, but any film-maker who pulls it off can expect to be underestimated. The Belgian brothers Jean-Pierre and Luc Dardenne are rightly celebrated as among the best working today, but because their films are so spare and straightforward, these former documentarists are sometimes thought of as still essentially plying a documentary trade, reporting on life in deprived corners of industrial Belgium.

The Dardennes' new film, The Kid With the Bike, reveals them at their best and as they really are – superbly economical storytellers who happen to work, with precise craftsmanship, using dramatic materials that their local terrain provides. Since their 1996 feature The Promise and its follow-up, the Palme d'Or winner Rosetta, the brothers have created taut, compelling fictions set around the declining steel town of Seraing. The Kid With a Bike is their most approachable and most upbeat film to date – and as narratively satisfying as Oliver Twist – which in its own pared-down fashion, it faintly resembles.

The kid is 11-year-old Cyril (Thomas Doret), living in a children's home since his absentee father dropped him there, with, it becomes clear, no intention of ever collecting him. We first meet Cyril raging to get through to his father Guy on the phone, and refusing to take a disconnected signal for a no. He won't accept that Guy has not only abandoned him but also sold his beloved bike. In one of his desperate escapes from the home, though, Cyril not only gets the bike back but acquires a surrogate mother.

Ducking into a doctor's surgery, he bumps into – literally bumps into – a young woman whom we barely glimpse at first. But soon after, she re-enters the picture: she's Samantha (Cécile de France), a hairdresser who has taken an interest in the boy and agrees to have him stay with her at weekends. We never find out why she's made this decision, and we don't need to. A mainstream film would have distracted us with a spurious backstory about Samantha's own deprived childhood, or whatever – but that's not the Dardennes' way. Samantha and Cyril have connected, that's all: for the Dardennes, it's a given fact, the sort of brick they build their stories on.

Samantha takes Cyril to see his father, who's in a hurry to give the kid the brush-off. Because the Dardennes like working with the same actors repeatedly, the casting of Guy is resonant: Jérémie Renier has himself played various kids with bikes in previous Dardennes films. He was the son of a slum landlord in The Promise, then a shiftless chancer who sold his baby in The Child. Renier is superb as Guy, pitiful but very human in his own childlike vulnerability. By casting him, the Dardennes not only work in echoes of those other characters but also indirectly suggest the man that Cyril might turn out to be – or not to be, if Samantha can help put him on a new path.

As it is, Cyril falls in with a small-time hood named Wes (Egon Di Mateo), and as a result, does something incredibly foolish that seems certain to make his new stability unravel. In fact Cyril is given a reprieve, but in a characteristic Dardennes move, the path to happiness – symbolised by a bucolic bicycling idyll – must take a surprising detour. We're left with an open ending that some might find infuriatingly enigmatic, others richly suggestive. Either way, you don't see it coming. If there's a happy ending, it's implied rather than delivered – let's say the Dardenne brothers make you work for it, or at least, imagine it for yourself.

The brothers have a record of unearthing raw young talent – Renier, Rosetta's Emilie Dequenne, and now Thomas Doret. His furiously energetic Cyril, driven by need for a father, for acceptance and – once he's on that bike – for sheer propulsive motion, is not so much a character as pure will. It's one of the best, rawest child performances I've seen since Kes – and the film's in that league too.

Dardenne purists might be shocked at the casting of Cécile de France, a mainstay of French commercial cinema, also recently seen in Clint Eastwood's Hereafter. But De France, who's actually Belgian, rolls her sleeves up for a tough, no-nonsense performance (watch Samantha dealing with a refractory boyfriend) and is a natural fit with the Dardennes' world. She also brings a tinge of down-to-earth glamour that makes Samantha a sort of dream mother to Cyril – a working-class version of Pinocchio's Blue Fairy, with DIY highlights and muscular arms. De France's Samantha and Doret's Cyril don't have it easy, don't even get on for much of the time, but they're an indomitable match made in heaven – a solid duo in a film by the most solid duo in European cinema.