As always with children's books, I make the disclaimer that I have no idea if the book is any use with children.
However, I thought the story of Virginia as told by her sister was charming and found a compassionate way of telling children about moods and people who may be affected by them.
The characters are obviously based on Virginia Woolf and her sister Vanessa Bell, which made me wonder whether the book tries to introduce the topic of depression but I think that was just my own association with the characters. The story itself stays at a more general level.
There is a nice twist to the illustrations at the end.