Majid Majidi’s cast of young toughs digging for treasure under a school deliver a heart-rending story with unexpected depth of emotion

Sun Children, by the Iranian director Majid Majidi, gives us a prison-break drama that is escaping to nowhere, and a knockabout school comedy gone horribly wrong. The acting is broad, the plot gears often creak, but it has guts and heart and a grubby, street-smart charisma. It’s one of the finest films playing in this year’s Venice competition.

Dedicated to “the 152m children forced into child labour”, this casts 12-year-old Roohollah Zamani as Ali, the pint-sized boss of a gang of thieves, a miniature wheel inside a much bigger machine, working for an unnamed local crime boss who skulks on the rooftop amid his pigeon coops. The boss wants Ali to retrieve a hoard of unspecified treasure, which is either buried in the local graveyard or in the drainage pipe that runs beside it. And the only way he can do it is to go back to school.