Who is Tanya, Misha from The Madonnas of Leningrad: A Novel and what is their importance?

Tanya is Viktor and Nadezhda's daughter, and Misha is their son. Tanya is suspicious and practical, Misha is a little spoiled and quite emotional. They are sent from Leningrad as part of a wave of children evacuated in the early stages of the siege and are expected, by Viktor at least, to return within a couple of weeks. As those weeks drag into months and then what feels like forever, Nadezhda becomes more volatile and more upset, coming to believe her children are dead. Her determination to celebrate Misha's birthday with a long forgotten piece of chocolate sends Marina out into the winter to retrieve that chocolate, and to one of the work's more significant contemplations of its thematic exploration of loyalty.