Burying the Sun by Gloria Whelan

Too young for the army, one boy takes saving the city into his own hands. The Russian city of Leningrad is darkening with winter and war, and Georgi's family prepares for the worst. His sister, Marya, packs up the great artwork at the Hermitage museum for safekeeping, and their mother tends to the wounded soldiers. But at fourteen years old, Georgi is too young to join the army, and he wonders how he can possibly help his friends and family. As the city slowly starves from lack of food and hope, Georgi knows he can help his people survive, but he must face dangers as real as the battles on the front lines.