Silent night - Mary Higgins Clark

A young mother from the Midwest travels to New York during the Christmas season to be with her husband who lies desperately ill in hospital. She has brought with her their two young sons, hoping to salvage for them at least some of the joy of the holidays. She, however, is rapidly reaching a point of despair.
While watching a street musician near Rockefeller Center's famous Christmas tree, Brian, the younger boy, sees another woman take his mother's wallet, and with it, not only all their money, but a precious family memento his grandmother has just given them, a St. Christopher medal which saved her husband's life in the war, and which she believes will save that of Brian's father.
Without telling his mother what he is doing, Brian follows the thief into the city's subway system, thereby beginning a journey that will forever change not only his life, but that of his mother and the thief as well.