Take a look at her,’ the narrator of The Twits says. The reader’s eye flits from the text by Roald Dahl to Quentin Blake’s illustration and then back to Dahl. ‘Have you ever seen a woman with an uglier face than that? I doubt it.’ And Mr Blake allows no room for doubt. Mrs Twit’s mouth is a scrawl of bad teeth, and a single baleful eye looks out from underneath hair that is an angry mess of scratched black lines.

This summer, the Twits join a gallery of witches and giants at the Museum of Childhood for an exhibition of Mr Blake’s unmistakeable illustrations to Dahl’s books for children. There is an exuberant roughness to Mr Blake’s art, the colours often spilling out over the lines. A marvellous film in the exhibition shows him at work in his studio, deftly sketching away with a stroke of ink here and a wash of watercolour there. More than 80 original drawings are on display, a treat for young and old alike.

The exhibition title, ‘Snozz-cumbers and Frobscottle’, was chosen by Mr Blake himself. As fans of The BFG will recall, snozzcumbers are the disgusting-tasting vegetables that the Big Friendly Giant eats instead of human ‘beans’, while frobscottle is his delicious if wind-inducing tipple of choice. As millions of pleasurably unsettled children can testify, Dahl always took more glee in the dreadful than the delightful. Yet Mr Blake’s illustrations have the effect of lightening the darkness.

When the prince in Roald Dahl’s Revolting Rhymes decapitates Cinderella’s cruel sister, Blake shows her pony-tailed head harmlessly and bloodlessly popping up into the air. Dahl’s text would often change in response to challenges faced by Mr Blake. In The BFG, an early reference to the giant’s long leather apron was removed as it would have to be featured in every illustration and was getting in the way. Mr Blake struggled to capture the giant’s footwear until an oddly shaped brown paper parcel arrived in the post. Inside, and now on display in the exhibition, was a contraption of thick leather straps one of Dahl’s own Norwegian sandals. This is what the BFG wears, said the accompanying note, and Mr Blake happily got to work.

By the time of George’s Marvellous Medicine in 1981, author and illustrator had developed a symbiotic relationship. Mr Blake has likened his illustrations to ‘a sort of handwriting’, and such is the success of their collaboration, that the reader moves effortlessly from Dahl’s lines of words to Mr Blake’s line drawings. ‘It is Quent’s pictures rather than my own written descriptions that have brought to life such characters as The BFG,’ Dahl once generously remarked. Vile villains, heroes and heroines such as Charlie and Matilda, plus animals of all shapes and sizes, live again in this hugely enjoyable exhibition. In the words of the jug-eared Big Friendly Giant, ‘it’s a razztwizzler’.

‘Snozzcumbers and Frob-scottle’ is at the V&A Museum of Childhood, London E2, from May 2 to September 6 (020–8983 5200; www.vam.ac.uk)