The kids aren’t all right in The Boys Are Back

Kid in Action has it easy. Usually running, usually screaming – the child actor playing KiA is basically at bigger, louder nursery school. KiC (Kid in Comedy) doesn't have too hard a run either – just look cute, supply sass and say the words. Let nice uncle editor take care of comic timing.

Kid in Drama (KiD) works in a scarier playpen. He/she needs to hold an audience through more than hollering and hamming it up. Especially since the last 20 years have seen a number of bar-raising performances from child stars in prominent roles – Natalie Portman in Leon, Haley Joel-Osment in The Sixth Sense and Keisha Castle-Hughes in Whale Rider, to name an obvious few.

If the child's role is too big for the actor, the whole film feels baggy. It's a problem that's so far swamped two movies at this year's London film festival - John Hillcoat's adaptation of Cormac McCarthy's The Road and Scott Hicks's family drama The Boys Are Back. Both The Road (starring Kodi Smit-McPhee, alongside Viggo Mortensen) and The Boys Are Back (in which then six-year-old newcomer Nicholas McAnulty stars with Clive Owen) hinge on the death of a mother in the first act. Before this event Smit-McPhee and McAnulty are in their natural territory – bit-part players acting as another facet of their fictional parents' relationship. With the mother gone, the weight of sustaining a believable family setup falls on the inexperienced child actors, and the responsibility is often too heavy for their small shoulders to bear. 

Smit-McPhee in The Road is cast as a famished nomad staggering through a world burnt to the brink of apocalypse. Of the few human survivors left, most have turned to cannibalism, as all plants and animals have died. The boy's mother – horrified by the world she has brought her child into – has killed herself, leaving the boy to her husband (played by Mortensen). Now father and son ("The Man" and "The Boy") trek a ruined highway, never sure if the next person they see will greet them, or eat them.

Smit-McPhee's role asks a great deal – he must sustain the look of a starving, desperate, bewildered child through 119 minutes in which he's rarely off camera. He manages bewildered, but looks as fit, healthy and alive as any normal child brought up in a secure, loving home. A key scene in which The Boy drinks his first can of fizzy drink is cut off from its poignancy by the fact that Smit-McPhee looks like he's scoffed a Twinkie just before they called "Action!".

In The Boys Are Back, Artie (Nicholas McAnulty) has been left alone with his sports writer father Joe (Clive Owen) after his mother suddenly dies of cancer. McAnulty and Owen share the screen for the first hour of the movie, with Owen as Bereaved Dad using the bored-looking McAnulty as a giant human tissue during scenes of Oscar-chasing hug and blub. Even Owen, normally an experienced and reliable draw, struggles with the ropey script. McAnulty, faced with lines such as "I want to die so I can be with mummy", just hasn't got a chance.

Some would say it's unfair to ask a 13-year-old to starve himself for his art, or to expect a six-year-old to understand and master grief, anger, and oedipal rage in his debut film. But these movies live and die on their young stars - we need to believe them to believe the story. There's a lot of competition out there, so they need to grow up and play the game. This is Hollywood KiDs.