Samuel Dylan Rosner was just 2 years old when, as his parents told the Times, they noticed him singing hymns from Passover in their entirety. After cutting his teeth in musicals like Annie, Fiddler on the Roof, and Seussical at the Jewish Community Center of Mid-Westchester, he made it into the Met’s children’s chorus. Now 9, he was a featured soloist last winter in the opera’s production of Prokofiev’s War and Peace.