Only a very brief biography is available on this British artist - Cecil Leslie (1900-1980) - even though she had many fine art exhibitions in the twenties and thirties as well as illustrating many children's books in the fifties and sixties: https://en.wikipedia.org/wiki/Cecil_Mary_Leslie
The author, Pauline Clarke (1921-2013) is using the name Helen Clare for this and all the Five Dolls books that were published between 1953 and 1963. https://en.wikipedia.org/wiki/Pauline_Clarke