“Only fuckers work,” declares the anti-hero (Jérémie Renier) of Belgian brothers Luc and Jean-Pierre Dardenne’s taut, stripped-down drama of urban dispossession, L’Enfant. Indeed, why work when you can steal and scrounge? And, come to think of it, why steal and scrounge when you can sell off your newborn son to the black market? (To which infant the film’s title refers is, yes, a matter of interpretation.) Here the street bandit’s girlfriend (Déborah François), otherwise so supportive of his scrappy entrepreneurialism, must draw a line. And so, from these simple essentials, the Dardennes render their slice of grim life, a film of impressive narrative economy and momentum. They make understated scripting, agile handheld camera work and commanding, unassuming performances look easy. Alert and tensely active even in its starkest moments, L’Enfant alludes to some great moments in French-language film history, and in retrospect may prove to be one.