The Lion, the Witch and the Wardrobe by CS Lewis
CS Lewis' classic story of four siblings―Peter, Susan, Edmund, and Lucy Pevensie―who are sent to a country house during the WWII bombings, and discover the magical land of Narnia when they step through a wardrobe door. The land is frozen in an eternal winter, and at the mercy of the evil White Witch - and so they unite with Aslan the Great Lion to fight the witch and bring hope back to Narnia.
This is a book I return to often. In this particularly challenging time, I think there is an enormous comfort to be taken from revisiting classics. When reading to young children, I think turning to old favourites can really help to pass on a love and enthusiasm for reading - if children can sense your enthusiasm for a story, I think they often respond very positively.