Human suffering is one of literature’s enduring themes. When it involves the agony of children it becomes heightened and enters the realm of the unimaginable made all too real by war and famine, which show no pity for the innocent. These two new novels by Spanish writers born only six years apart place children in appalling predicaments.

In that of the younger, The Boy Who Stole Attila's Horse, by Iván Repila – born in 1978 – two brothers, Big and Small, are trapped in a well with no hope of escape. Hunger drives Small to madness as death hovers over them. Only one will survive.

Out in the Open, by Jesús Carrasco – born in 1972 – is about an abused boy who has run away from home. Initially he hides in a tiny hole in the ground, listening to the search party calling his name. As the voices retreat he takes his chances and begins to plan his escape across a vast, arid plain. In addition to his pursuers he must deal with hunger, heat and the brutality of adults.

Neither is an easy book. Both are ambitious and relentless, sparing neither their characters nor the reader. Yet the difference between the two is devastating. The Boy Who Stole Attila's Horse (translated by Sophie Hughes; Pushkin Press, £10) is high art, an imaginative allegorical work of breathtaking yet restrained lyric power: " 'It looks impossible to get out,' he says. And also: 'But we'll get out.' " The well is compared with "an empty pyramid with no tip".

Repila’s prose is clinical, precise and beautiful. His sinuous, lilting Spanish has been magnificently rendered into English by Sophie Hughes in a faultless, rhythmic translation that enables the bleak narrative to soar like an epic poem.

This exciting book casts an awesome spell; it is a metaphysical fable brought to life. The traditional European fairy tale has been born anew, laced with an even darker realism. “Small dreams about a swarm of butterflies and watches himself catch them with his long, retractable tongue.” The well becomes a battlefield, larger than the universe. Nature’s beauty is juxtaposed with the more repulsive aspects of decay.

By contrast

the most daunting challenge facing the English-language reader of Carrasco’s narrative is the leaden prose. Already billed as an “international bestseller”, and set to be published in 20 countries, this is a laboured, determinedly literary novel, heavy with weighted imagery: “The desolate howling of fire-scorched scrub.” What does that mean?

Carrasco favours wordy over statement. The boy recalls his previous life: “His father would arrive, breathing hard, initially thrilled and happy. They would form a whirlpool of people around him that would barely let him breathe, like a newly struck match that struggles at first and shows no sign of becoming the mellifluous flame that will eventually consume the matchstick. They would disinter him amid shouts of joy.”

The drama of the action, a chase, is undermined by repetitive descriptions of smells and references to exhaustion, thirst and hunger. There are the inevitable dead trees, foul water, the need to urinate, the act of urinating, his responses; the boy appears to move in slow motion. Each movement is explained with a zealous, overly detailed description, making the narrative appear far longer than it is or needs to be.

It certainly does drag on, and all too often Carrasco loses his intended effect through cliches: “The sky was a dark, dark blue. Up above, the stars were like jewels encrusted in a transparent sphere.”

The formal, stilted language is a problem; an even larger one is the stylistic presence of Cormac McCarthy’s apocalyptic influence. Yes, the boy meets a lone, stoic, quasi-saintly goatherd, aged and weakened, though resourceful. He helps the boy and symbolically steers him further into the harsh world of adults. The old man is forgiving yet not without anger.

At times the comparisons with McCarthy overpower the narrative, as do the baddies; it is difficult to accept that the evil bailiff, a brutish individual indifferent to the censure of others, would expend so much effort tracking down one of the apparently many boys he had abused.

It is not a fairy tale, and at no time does Carrasco appear to see the reader as more than passive, with nothing left to the imagination. There are some irritating points of logic. Early in the narrative, for example, while still crouched in his earthen pit, the boy recalls a crippled man “who moved about the streets on a kind of tricycle propelled by a handle that he turned, bending over it like an organ-grinder”. He had many dogs and would “travel the beaten paths heading north, the only ones his chariot could manage”.

So helpless was the man that the boy “often wondered why he didn’t get the dogs to pull him”. That seems to make some sense. But, straining for drama, Carrasco adds that the boy had been told that the cripple used to hang the dogs from an olive tree when he had no further use for them. The obvious reaction is to wonder how a man so physically challenged could suspend dogs from trees.

If the writing were better the story might well distract from the illogic. But it doesn’t. The boy recalls hunting rabbits. “Then, without mentioning his mother, he described, as if they were his own, his techniques for skinning a rabbit.” The goatherd explains that hunting is pointless, as the fire needed to cook meat would attract the men pursuing them. The boy sets off, returns with a rabbit and the old man cooks it – and all within a few sentences.

Reading Out in the Open (translated by Margaret Jull Costa; Harvill Secker, £12.99) is akin to being in a diving bell; it is very much at a remove from cohesion. It is surprisingly difficult to engage with it, as it reads like a painstakingly constructed performance piece too fraught to have taken the time to consider true emotion.

The stage is littered with props, but the prose fails to convey more than a superficial flow of words. For all the effort it is cold and predictable, particularly in its grotesque flourishes. For all the realism it is impossible to believe in the story.

But The Boy Who Stole Attila's Horse explores through an artist's eye what it means to be human. With echoes of Agota Kristof's The Notebook and Jim Crace's Being Dead , Repila guides the spectral brothers through the seven circles of hell as they encounter fear, hunger, madness, rage, acceptance of death and the grief of survival.

After weeks of starvation they fight over a dead bird, aware that eating it will kill them. All existence is experienced within the well. Repila, a seer possessed of daunting vision, alerts us to the possibilities of fiction, its inherent artistry. This exquisite, terrifying novella is daunting and magnificent, a book that celebrates storytelling as the truest way towards understanding existence.

Eileen Battersby is Literary Correspondent