The children, however, came as a surprise to his wife Eliza. She had none - and his long-dead first wife had had none either. Cruikshank had revealed his secret. Three streets from their home in Hampstead Road, north London, he had another household where the love of his life had borne him 11 children.
Devastating news for any wife, but especially for Eliza. Cruikshank was known, not just as a famous artist, but as a model of Victorian rectitude - a teetotal campaigner for 30 years who would denounce his early indulgence in 'late nights, blue ruin (gin) and the dollies'.
The deathbed scene in 1878, and what followed, have only just come to light, as an indirect result of an exhibition I have organised to celebrate the bicentenary of Cruikshank's birth.
Who was his secret love and what became of her and her children? Until recent years, the power of shame and guilt has caused such women to be written out of history - as Dickens's mistress Nellie Ternan was until Claire Tomalin recently published her biography, The Invisible Woman.
I found that Cruikshank's mistress, Adelaide, died in Croydon in 1914, so I tried a long-shot: a few inquiring paragraphs in the Croydon Advertiser. The result was that I discovered Hazel Snaith, a great-granddaughter of Adelaide's sister.
Thus I learned that Adelaide was born in 1831 at Claygate, Surrey, daughter of a farm labourer. Soon after Cruikshank married Eliza, then 43, in 1850, this girl with blue eyes and golden ringlets came to 263 Hampstead Road as their maid.
In 1853, two things happened that can be seen as liberating for Cruikshank: his mother, a stern Presbyterian died; and he, aged 61, began visiting Adelaide in her attic room. 'He forced himself into her bedroom,' is how Mrs Snaith puts it. Her daughter, Jacqueline Owen, who lives in Croydon, says: 'He was smitten with her.' It could have been her ruin, but she emerges as a survivor.
The story can be told because Adelaide spent her final years in Croydon in the house where Mrs Snaith's mother grew up. Some details survive because Jacqueline Owen, as a teenager a dozen years ago, persuaded Mrs Snaith's mother to talk about Adelaide.
At 263 Hampstead Road Adelaide became pregnant. Despite what Mrs Snaith calls 'a strong affection' between Eliza and Adelaide, the maid would not reveal that Cruikshank was the father: and a pregnant maid had to go.
However, Cruikshank set her up in a house about two minutes' walk away, at 31 Augustus Street, and thus a young woman who might otherwise have ended on the streets became ostensibly the wife of a mature gentleman, 'George Archibold'.
When census returns had to be filled in, his occupation veered between artist, commercial traveller and wood engraver. He fathered six daughters (one died young) and five sons, the youngest conceived when he was 82.
This stocky, broad-shouldered, combative man was always proud of his fitness. In his eighties, it was noted, 'he walked like a man still young'. He ascribed his robust health to the fact that he had been teetotal since he was 55 and had given up smoking. All his life, too, he was young in heart.
In Regency London he had won fame as a bold and sometimes scurrilous caricaturist, but he had gone on to create a vast body of work suitable for all the family, as comic artist and as illustrator for Dickens, among others. And he was venerated by the Nonconformist churches in particular as a high Victorian moralist. So what did Eliza do when, bending over the deathbed in February 1878, she learnt that George had a swarm of children by the maid of whom she had been so fond? She went round to Augustus Street, says Mrs Snaith: but not to play the outraged wife. For the first time in nearly a quarter of a century she saw Adelaide, and insisted on helping the mother of her late husband's children. Cruikshank had made some provision for them, but Eliza nobly added to it.
'She would do anything for them,' says Mrs Snaith - even helping to send the girls to finishing schools.
All that had to stay secret. To the public, Cruikshank had to remain a paragon. As Punch said in its obituary: 'There never was a purer, simpler, more straightforward or altogether more blameless man. His nature had something childlike in its transparency.' He is buried in St Paul's Cathedral.
He must have scores, even hundreds, of descendants. I have made contact with descendants of the first-born, George Robert Archibold, whose son became an engineer in charge of London's bridges. His granddaughter, Frances Hedges, aged 81, and her granddaughter, Joanne Purves, who is 24 and works in London, came to the recent London opening of my Cruikshank exhibition.
Joanne, Cruikshank's great-great-great-granddaughter, saw a picture of Adelaide for the first time: 'It's very romantic to have a story like that in the family,' she said. But where are the descendants of the other Archibolds, Adelaide (born 1858), William Henry (1860), Albert Edward (1863), Alfred (1865), Eliza Jane (1867), Ada Rose (1868), Emma Caroline (1869), Nellie Maude (1873), and Arthur (1875)?
When Mrs Snaith's mother was a girl, just before the First World War, she would often ask the aged Adelaide, 'Who was your husband, Auntie Adelaide?' And Adelaide, sitting in her rocking-chair, would always say, 'I can't remember, my dear.' Jacqueline Owen says: 'She had a little twinkle in her eye when she said it.'
The exhibition Cruikshank 200 will be at Towneley Hall Art Gallery, Burnley, Lancs, 15 Nov-24 Dec, and in January-May at Maidstone, Sheffield and Twickenham.