The Boy in the Box by Cary Fagan
Twelve-year-old Sullivan Mintz helps his family run the Stardust Home for Old People. While his father worries about finances and his mother is distracted by her poetry (rather embarrassingly, she's known as the Bard of Beanfield), Sullivan deals with his little sister, who can be a bit of a brat, and gets taunted at school for being short and clumsy. As a result, Sullivan's best (and only) friend is 81, which is pathetic, but at least Manny introduced Sullivan to his #1 obsession—juggling.
One night, Sullivan glimpses Master Melville's Medicine Show, a travelling caravan. Enchanted by its mysterious and magical allure, he has to visit it again. But thrill turns quickly to dread when he steps onstage for the magic show ... only to wake up trapped inside the caravan!
Sullivan's chances for escape are dashed by the Melvilles, but he's comforted by the other children in the show. Clarence, the small boy who trains dogs, becomes Sullivan's first friend his own age—and there's Esmeralda, the beautiful girl who walks the tightrope. Best of all, this new "family" values his juggling!
While Sullivan is coming to terms with his new life, his younger sister and Manny hit the road, intent on tracking him down. But even as they follow, the caravan rolls on, taking Sullivan and the others farther away from their homes.