Who is Beatrix Willow from In the Small and what is their importance?

Beatrix Willow is Mouse Willow's little sister. Her nickname is "Beat", and she was named after Beatrix Potter. When the blue light strikes earth, she and her mother are shrunk to one-twelfth of their normal size. Even though she is very young, Beatrix seems to take over and begins to display the same leadership skills as that of her older brother.