Who are the children in your head? The ones you can hear loud and clear whenever you think about them, I mean. Perhaps they’re your own children, or nephews, grandchildren or small friends. But I’d bet some of the most memorable of them don’t actually exist; never have existed in the flesh-and-blood sense. Their little limbs are made of words.

If you’re anything like me, your mind houses a ragged gang of them: valiant Scout from To Kill a Mockingbird, maybe; Twain’s crafty Tom Sawyer; Roddy Doyle’s awkward Paddy Clarke; the rational yet temperamental Alice in Wonderland. The child narrators of great novels never grow up, never grow old, and certainly never die. They keep their grubby-fingered hold on us long after we’ve forgotten everything else about the books that spawned them. The young protagonist of L. P. Hartley’s The Go-between, for instance; I couldn’t tell you much about the plot, but I’ll never forget the boy’s lingering confusion, in the overheated fragrance of summer, or his embarrassment about mishearing the name “Hugh” as “who”.

I’m using “narrator” in a broad sense, here, to mean a point-of-view character. Like most readers, I often can’t recall whether a book is written in the present tense or the past, the third person or the first or even the second; those are technical points for the author to worry about. All the reader demands is a moving and gripping perspective on the events of the story, and a sense of authenticity and immediacy. (Think of the sensory freshness of childhood in the opening pages of Portrait of the Artist as a Young Man.)

For my new novel Room, I never felt I had a choice. It’s about a five-year-old boy who has never known (or even known about) a world outside the 11 x 11 foot cell somewhere in America in which he lives happily enough with his beloved young Ma: the novel had to be entirely from Jack’s perspective. A woman’s story of kidnap, confinement, rape and motherhood would be too obvious a tear-jerker. What drew me to this material was the notion that, for the child, this could be an entirely different kind of story, more like a cross between science fiction, an adventure and a fairytale. So Room’s point of view was its whole point.

Novels with child narrators often fake it. I say this without laying blame: fiction is all a fake, in a sense. So authors often choose juvenile point-of-view characters who are eloquent child prodigies (Nancy Huston’s Fault Lines and Jonathan Safran Foer’s Extremely Loud and Incredibly Close) or whose childish insights are supplemented by adult wisdom, in brief flashes of hindsight (as in Michael Frayn’s marvellous Spies). For Room, I set myself a stricter task: I wanted Jack to have the thought processes of a normal five-year-old boy (despite the oddities of his upbringing). I decided to trust this small child in my mind’s eye; I had a hunch he could tell this strange story far better than anyone else.

Voice is a different matter, however. Of course I want my readers to feel they are hearing a real live American boy. But just as to give the impression of a 40-year-old Irishwoman narrator, what I would put down on the page would not be the actual, rambling, shambolic speech of a 40-year-old Irishwoman, so a child’s voice in a novel is a literary trick – a careful construction.

It was hard, but no harder than finding a voice that rings true for any other character; there are adults I would find it trickier to “channel” than five-year-old Jack, whether because I’m not familiar with their dialect or idioms or because their mindset is alien to me. So Jack came readily enough to me. And it was certainly a great help that my voluble son Finn was five when I was writing the novel.

But I never simply wrote down what Finn said, because I knew (from experience!) the raw, crazily ungrammatical stream-of-consciousness of a small child would drive any adult reader to distraction. So I mined Finn’s chatter for raw material, as it were.

The first decision I made was that Jack – who is far more skilled in conversation that most five-year-olds, since he’s been talking to his intelligent mother non-stop for years, while other children run around with water pistols – would have the gift of coherence. I would let me tell his story in a linear and fairly clear way. But the rule I set myself was that although Jack’s sentences would line up in a coherent way, each of those sentences should sound like something a five-year-old might say.

For previous plays and novels, I have put together mini dictionaries of, say, 17th-century peasant Hiberno-English, or 18th-century London street slang. I prepared for Roomthe same way: I charted Finn’s language, analysing every mistake of grammar or vocabulary. The list went on for pages.

Then I cut it down to a mere handful of the best mistakes: the ones that reveal the most about child psychology. One is the tireless attempt to make the past tense of irregular verbs consistent, as in “I eated” and “I winned”. Another is the flabbergasted piling on of terms of comparison (“way super bigger than the biggest ever ever”) and negation (“no I won’t not ever never no time”).

Every child has their own flavour, of course; there’s no such thing as a generic child’s voice. In Jack’s case, I wanted his first-person speech to reflect not only the standard American dialect he has learned from his Ma and from television, but the strangeness of his two-person society. I thought of him and Ma in tribal terms (rather than seeing them as freakish versions of ordinary Amercans), and decided Jack’s world view would be animistic: the seeing (and respecting) of a spirit in everything. So for him, every object in Room has a capital letter as well as a sex. Rug and Wardrobe are not just bits of furniture but comforting mentors; Jeep is less a toy than a friend; Eggsnake (strung together from the blown and decorated shells of every egg Ma and Jack eat) is the living embodiment of his childhood years.

Child point-of-view in fiction for adults is still the exception rather than the rule. Many authors avoid it – perhaps over-estimating its technical difficulties – and many readers shy away from it. Perhaps they fear the experience of reading such a novel will resemble being locked in a room with a small child ...

Luckily, for my purposes in writing Room, those occasional moments of claustrophobia suit the book, helping readers to imagine Ma’s situation, never more than a few feet away from a boy she adores but cannot escape. (I defy any parent to tell me they’ve never had those feelings.)

The wonderful paradox of child narrators is that they know so much less than adult ones, which can let them guess and imagine so much more.