Visitors to today’s Alcatraz Island off San Francisco tour the cell blocks, kitchens, hospital wing and laundry guided by the voices of past residents speaking through their headsets: not only inmates and guards but the guards’ families, including those who spent their childhood on the Rock during its 29 years as the most notorious prison in the US.

Alcatraz in the 1930s, the era of Al Capone, is the evocative setting for US author Gennifer Choldenko’s story of the Flanagan family, struggling to find the right care and treatment for teenage daughter Natalie, whose autism has yet to be recognised as a condition. This novel for upper primary readers and above is the unanimous choice of the judges for this year’s Children’s Book Award with its picture of the wider repercussions of special needs, alongside a tale of growing up and learning to break the rules in one of the most regulated communities in the world.

In the book our guide to Alcatraz is Moose, Natalie’s younger brother, with his engaging voice and sardonic humour: “You get to Alcatraz by being the worst of the worst. Unless you’re me. I came here because my mother said I had to.”

Moose’s life has been built around a series of family crises brought about by his mother’s desperation to find a “cure” for Natalie. While his sister has been prodded, prayed over and assessed, Moose has been shunted off to stay with grandparents, has waited with his parents for the latest bad news and tried to retreat from the emotional fallout at home.

When the Flanagans arrive on Alcatraz, Natalie is about to celebrate her 10th birthday for the seventh time (her mother knows that the professionals are not equipped to help adolescents). Her last resort is a progressive school in San Francisco so Moose’s father, whose desperation is more controlled than his wife’s but no less intense, is working double shifts as a guard to pay for it and the family has exchanged extended family and friends for a cramped apartment in the staff quarters, which can still be found on Alcatraz today.

When Natalie is sent home after one night’s stay, the Rock becomes her prison and Moose her reluctant guardian. Moose’s complaint about Alcatraz on a bad day - “nothing to do, no one to do it with and nothing to look forward to” - could be said to sum up Natalie’s powerlessness in her extended childhood.

While Moose pours out his resentment at missing baseball, leaving his best friend on the mainland, having to accommodate his sister in his new gang of guards’ children and being expected to be “responsible for everyone else in the world”, his love for Natalie, his anger at the world for rejecting her, and his growing ability to engage with her - he learns to appreciate her daily delight in watching the sun rise - is a moving strand in the story.

The Flanagans emerge from their troubles as a beleaguered, but strong family. The treatment of Natalie’s character, inspired by the author’s sister who had a severe form of autism, is the book’s key triumph. While Choldenko does not use Natalie’s voice or try to enter her head, she gives her a defined personality and makes her accepted in her own right by Moose’s friends, despite the initial prejudices of the warden’s daughter Piper who rules the junior wing of Alcatraz.

Natalie’s condition is not used as a plot device; rather it’s her ability to act independently and form a connection with a prisoner, which Moose finds inappropriate and disturbing, that unites the separate strands of the story. For a girl obsessed with counting, a well-ordered environment where everyone has a number is strangely appealing.

The reader also gets to share in the part of Moose’s world that is closed to Natalie. His mainstream high school should be his chance to blend in with his peers but he is pulled into the spotlight with the other “Alcatraz kids”, induced by Piper into spinning yarns about the inmates and drawn into her outrageous prison laundry scam, source of the funniest scenes in the book.

His parents’ response when the inevitable showdown comes explains Moose’s caution about breaking rules: the highs and lows of their concerns for Natalie mean zero tolerance for drama created by their son.

It’s not clear whether the satisfying breakthrough at the end of the novel will leave Moose free to stop being old beyond his years.

Much of the circumstances of the book are rooted in fact: Al Capone’s mother could have met Moose and his friends on the visitors’ boat; Al Capone did work in the prison laundry; the children of Alcatraz did sing Christmas carols to the inmates; children like Natalie in the 1930s had very little hope of appropriate treatment. The Flanagans are fictional characters but the way in which they adapt to their circumstances ring true, now as then.