The Secret of the Blue Glass, Tomiko Inui

On the first floor of the big house of the Moriyama family, is a small library. There, on the shelves next to the old books, live the Little People, a tiny family who were once brought from England to Japan by a beloved nanny. Since then, each generation of Moriyama-family children has inherited the responsibility of filling the blue glass with milk to feed the Little People and it's now Yuri's turn. 
The little girl dutifully fulfils her task but the world around the Moriyama family is changing. Japan is caught in the whirl of what will soon become World War II, turning her beloved older brother into a fanatic nationalist and dividing the family for ever. Sheltered in the garden and the house, Yuri is able to keep the Little People safe, and they do their best to comfort Yuri in return, until one day owing to food restrictions milk is in shorter supply...