The Clockwork Three by Matthew J. Kirby
Three ordinary children are brought together by extraordinary events. . . Giuseppe is an orphaned street musician from Italy, who was sold by his uncle to work as a slave for an evil padrone in the U.S. But when a mysterious green violin enters his life he begins to imagine a life of freedom. Hannah is a soft-hearted, strong-willed girl from the tenements, who supports her family as a hotel maid when tragedy strikes and her father can no longer work. She learns about a hidden treasure, which she knows will save her family -- if she can find it.