LONDON • Here is a marvellous tale to jumble your winkles: Once upon a time, there lived a man who was almost 2m tall - a big, sometimes friendly, swashboggling giant of children's literature.

But here is the most scrumdiddlyumptious thing of all - it was this gobblefunking gentleman who conjured two of the greatest girl- heroines in the history of literature.

One, a five-year-old genius with telekinetic powers and a love of Charles Dickens, the other a small and bespectacled orphan who enlists the queen of England to help quash a pack of giants.

The eponymous Matilda and The BFG's Sophie are, of course, the indelible creations of the late Roald Dahl.

The writer, who died in 1990, has a splintered reputation. He is Britain's national treasure, the magical man who brought people some of the best-loved books in the history of children's literature. He was also, by most accounts, a bully, a bigot and a womaniser.

Perhaps our unwillingness to reconcile these elements of his being - the nasty and the nice - explains why he is so rarely absent from the cultural conversation.

But, as any kid knows, it is the icky, sticky, often delicious blend of nasty and nice that makes his books tick.

With The BFG (Big Friendly Giant) - now a Steven Spielberghelmed Disney film - Dahl's words, alongside Quentin Blake's drawings, remain a perfect, inseparable match: those naif, febrile and scratchy little sketches, paired with Dahl's arch and antic tone, in which something fractious and faintly peevish always simmers beneath the sweetness.

With Sophie and Matilda - both scrupulously polite, conscientious little girls - that simmering is one of revenge. Matilda must contend with the tyrannical headmistress Miss Trunchbull, who has swindled the shy and lovely Miss Honey out of her inheritance, while Sophie is up against a pack of child-eating giants.

At one point, Matilda's mother gabs: "Looks is more important than books."

Matilda, of course, proves the opposite to be true: Both she and Sophie succeed through their wily intelligence.

Even that brute Trunchbull knows that "a bad girl is a far more dangerous creature than a bad boy. What's more, they're much harder to squash. Squashing a bad girl is like trying to squash a bluebottle. You bang down on it and the darn thing isn't there".

Generations of small and unsquashable girls have internalised the lessons of Sophie and Matilda, to learn that smarts and cunning prevail over force and fury.

Matilda, as the Slate's critic Chelsey Philpot put it, "made being a nerd cool before being a nerd was cool". Another woman tweeted recently that the book "let me know that my thoughts were indeed valid; even as a child, nerd, girl, outsider etc..."

Of those four designations, Dahl himself could lay claim only to the first: He was, for better and for worse, a permanent child.

In old age, he would refer to himself as a "geriatric child".

His granddaughter, model Sophie Dahl - on whom The BFG's heroine is based - has spoken about burying him with an array of his favourite things, including: "The most enormous box of chocolate bars, Twixes, Kit-Kats, Mars bars, everything, just to keep him going 'til he got to heaven."

In his children's stories, Dahl championed underdogs and loathed bullies. The pluck of Matilda, for example, is enhanced by the grotesquely wrought, towering Trunchbull.

With her "bull neck", grapefruit calf muscles and propensity for defenestrating boys and hurling girls by their pigtails, she is matched in horridness only by the giants of The BFG - Bonecruncher, Fleshlumpeater, Bloodbottler et al.

Off the page, however, Dahl could be just as rotsome as his monsters. He was an egregious and unashamed anti-semite who, in 1983, sought to defend his prejudice with some appalling - and appallingly misjudged - words.

"There is a trait in the Jewish character that does provoke animosity," he told the New Statesman. "Maybe it's a kind of lack of generosity towards non-Jews. I mean, there is always a reason anti- anything crops up anywhere; even a stinker like Hitler didn't just pick on them for no reason."

Eventually, his bad behaviour became intolerable to Robert Gottlieb, Dahl's long-suffering - and Jewish - editor.

When Gottlieb sent Dahl a letter washing his hands of him, he allegedly received a standing ovation from the rest of the staff at the Knopf publishing house.

"You have behaved to us in a way I can honestly say is unmatched in my experience for overbearingness and utter lack of civility," Gottlieb wrote. "I've come to believe that you're just enjoying a prolonged tantrum and are bullying us."

Appreciating the pragmatic heroism of his girl characters is also easier if you've never read Dahl's adult fiction. For grown-ups - or, at least, ostensible grown-ups - he wrote nasty tales oozing with bodily revulsion and sexual humiliation. Playwright Noel Coward, writing in his diary, observed in them "an underlying streak of cruelty and macabre unpleasantness, and a curiously adolescent emphasis on sex". It seems more accurate to call that streak "blatant" rather than "underlying".

As Dahl himself got older, his stories did indeed become sweeter and wiser. Published in 1982 and 1988 respectively, The BFG and Matlida constitute the last children's books he published in his lifetime.

Decades on, you can see Sophie and Matilda's lineage in a succession of quietly valiant heroines, among them Lemony Snicket's Violet Baudelaire, Philip Pullman's Lyra Belacqua and J.K. Rowling's best and most bookish muggle- born witch, Hermione Granger.

Through them and their antecedents, small minds such as Matilda's continue to grow, "nurtured by the voices of all those authors who had sent their books out into the world like ships on the sea".

Even a grown woman might, as she does, receive a piece of solace emptied out of nastiness: "These books gave Matilda a comforting message: you are not alone."

THE GUARDIAN

• The BFG opens in Singapore on Aug 18.