Revealing another side to the much-loved illustrator of classic children’s novels such as The BFG and The Twits, Sir Quentin Blake’s series of watercolours of naked mothers with their babies feature in an assortment of pictures from the acclaimed illustrator’s personal collection which are due to be auctioned for charity next month.

The images, drawn for the delivery room at the university hospital in Angers, France, show the women swimming underwater with their babies, among seaweed, or breastfeeding. They form part of a collection of 178 illustrations from Blake which Christie’s will auction between 3 and 12 July to raise money for The House of Illustration, Roald Dahl’s Marvellous Children’s Charity and Survival International.

“They’re of mothers and babies swimming about underwater, of course they’ve got no clothes on,” said Blake, the UK’s first children’s laureate and the winner of the 2002 Hans Christian Andersen award for illustration. “l hope they’re joyful.”

Covering four decades of Blake’s career, the illustrations range from his portrayal of the long-lost Beatrix Potter character, Kitty-in-Boots, to his illustrations for David Walliams’ Mr Stink, and the first book he illustrated for Roald Dahl, The Enormous Crocodile. A mix of what Blake calls “alternative versions” and preliminary drawings, the collection also includes Blake’s initial painstaking attempts to get the BFG’s ears right – which Dahl described as “a bit preposterous”.

“It was him working out how big the ears should be,” said Christies’ Sophie Hopkins of Blake’s illustrations. “There is always a gulf between the author and the illustrator, and in this case the first version wasn’t quite right, and Roald Dahl sent it back. The drawings give you an insight into how he is constantly trying to create the character shown on the page.”

“I get very concerned about whether this is a better likeness than that. Sometimes after I’ve done it I can’t even tell later why I chose that one – and sometimes I think I chose the wrong one,” said Blake, in a video interview for Christies about his “alternative versions”. “When I’m doing the roughs I’m thinking I am that person. It is like performing it. The roughs are the rehearsals and then you have to go on and do it … I start with the face because if I think that isn’t quite right I’ll leave it there and start another one.”

Alongside a raft of illustrations for Dahl’s writing, from Mrs Twit to George’s Granny, the collection also features pencil drawings from Blake’s exhibition Arrows of Love, showing nude women avoiding or embracing Cupid’s arrow. Christies said the “rarely-seen nudes reveal Blake’s personal reflections on the joy, folly and sorrow of love”.

“What unifies his work is that wonderful line that captures the character he’s drawing – seemingly effortlessly, but the rest of us can only look on in awe,” said Hopkins. “There is so much affection captured in his drawings of mothers and babies – while they’re different in intent to his illustrations for Roald Dahl or David Walliams, what is really great is his drawing is so recognisable. That sense of mischief is what defines Quentin’s work – he’s a man with an eternal twinkle in his eye.”

Estimates for the drawings range from £200 to £10,000, with 30 of the illustrations to be presented in Christies Valuable Books and Manuscripts auction on 11 July, with a dedicated online sale of the remaining 148 to be open for bidding from 3-12 July.