Code name Kingfisher / by Liz Kessler.

Enhanced descriptions from Syndetics:

A young girl learns of her grandmother and great-aunt's involvement in the Dutch Resistance during World War II in this heartbreaking middle grade story of family, history, resilience, and hope from acclaimed author Liz Kessler.

Thirteen-year-old Liv's beloved ninety-two-year-old grandmother, Oma, is moving into a home where she can be cared for as her dementia worsens. As Liv helps her father empty Oma's house, she finds an old chest which opens up a whole world that Liv never knew about: the hidden world of Oma's childhood.

Through the letters and other mementos, Liv learns that Oma, given name Mila, had a sister, Eva, that no one in Liv's family ever knew about. In 1942, Mila and Eva are sent away from their parents to a non-Jewish family so they will survive the war. Twelve-year-old Mila believes that they will soon be reunited with their parents and go back to their normal lives, but fourteen-year-old Eva knows better, and soon gets involved in the Resistance. Eva takes on more and more dangerous assignments until a betrayal forces her to decide between running away with her sister or fully committing to mission. Tragedy strikes, and Mila goes to England on her own to restart her life from scratch, vowing never to talk about her childhood again.

In the present day, Liv reads how Mila builds something new from the shattered pieces of her childhood while giving beloved Oma all the support she can. Both Liv and Mila grapple with loyalty, family, and love as they discover what it means to be brave and go above and beyond to offer someone else a life of dignity, happiness, and freedom.