Growing up in the Hongkou Ghetto, an area designated for Jewish people by the Japanese occupiers, Sonja was a lively girl who "never forgot anything". She recalled her mother reading her the German fairy tale of Snow White. When it snowed one day, her father gathered the snow from the roof and put it in a basin, so that she could play with it. A lot of historical documents were kept in the family, such as her parents' boat tickets to Shanghai, her vaccination card and other items, which all became important elements for her book.
Her brother was only 2 when they left Shanghai in 1947, and he once asked about his sister's passion for Shanghai and China. Still calling herself a Shanghainese, and considering it her birth home, Muehlberger made altogether nine trips back to the city since 1998.
Her childhood in Shanghai gave her an open personality, ready to engage with people from other cultures. She recalled with excitement the first time she saw Chinese people after the family moved to Berlin. It was in 1951 and a Chinese delegation of children participated in the World Youth and Students Festival there. She made friends with the Chinese Young Pioneers of her age, and they exchanged red scarves.
