The Thief Lord by Cornelia Funke, 2000.

Prosper and Bo (short for Boniface) are orphans, running away from their caregiver. They were living with their aunt, a wealthy woman who likes little Bo and treats him like a little doll but doesn’t like Prosper because he is too old to treat like a toy. When the aunt starts considering sending Prosper away to boarding school, they run away to Venice to hide. There, they meet up with a band of orphans who live in an old movie theater. The other orphans/homeless children are Riccio, Mosca, and Hornet. Each of them is running from something in their past, and lacking adults to take care of them, has made their home in the theater, stealing and picking pockets to survive. The Thief Lord, the leader of this group, is the mysterious Scipio, a boy with his own secrets.

Many of the adults in this book are varying degrees of unhelpful and threatening as the children struggle to take care of themselves and their own lives. The boys’ aunt hires a detective to find them, and he does eventually figure out where they are living. Scipio turns out to be the son of the wealthy man who actually owns the disused theater. However, in spite of his father’s wealth, he does not have a happy home life. His father ignores and neglects him because he is too obsessed with his business concerns. At one point, the boy’s beloved cat is very sick, and Scipio fears that she might die, but his father brusquely tells him that he has no time to take it to a vet, and he’ll just buy him another one. (Don’t worry, the cat gets saved.)

One of the thefts that Scipio has the children attempt is the theft of a lion’s wing owned by a woman named Ida Spavento. A man known as the Conte, a customer of the man who buys the band’s stolen goods, specifically requested this item, although the children do not initially know why. Ida catches them in the act and tells them that the wing was once part of a magical merry-go-round that can change the ages of the people who ride it, making older people younger and younger people older. When Ida learns that the children received a request to steal the lion’s wing, she allows them to have it on the condition that she is allowed to come with them to see who hired them.

The Conte and his sister have discovered the merry-go-round and need the missing wing to complete it. Once the wing is restored, the merry-go-round turns them into young children. When Scipio sees that the merry-go-round really changes a person’s age, he insists on riding it himself. After a ride on the merry-go-round, Scipio becomes a young adult, able to live independently from his hard-hearted father and take care of his beloved cat. Barbarossa, the man who fences their stolen goods, also rides the merry-go-round and becomes a little boy again, far more adorable than the man he used to be. However, he accidentally breaks the merry-go-round so that the magic no longer works.

With the exception of Ida and the detective, Victor, the other adults in the story are irresponsible and self-centered, really more like children than the children. When the merry-go-round changes the ages of some of the characters, the changes end up suiting their personalities. Scipio was always more mature, and his new status as an adult reflects that. He ends up becoming an assistant to the detective in his agency, who understands the whole situation. Ida adopts Prosper, Bo, and Hornet and takes care of them. Prosper and Bo’s aunt is charmed by the now adorable Barbarossa and takes him in. Barbarossa, who is as shifty as a child as he was as an adult, is more than happy to let the aunt pamper him and fuss over him because he knows that he’ll get to enjoy her money in the process. The end does reveal that the aunt catches Barbarossa stealing from her and sends him to boarding school for some discipline, but he bullies and dominates the other boys into doing his bidding and even stealing for him, becoming the new Thief Lord.

The book is available to borrow for free online through Internet Archive.