Complex sentence: In 1930 she illustrated Marion King's "Kees", and Nellie Marie Rowe's "The Crystal Locket" in 1935, then she began writing and illustrating her own children's books in the 1930s and' 40s.
Split into simpler sentences: In 1930 she illustrated Marion King's "Kees", and Nellie Marie Rowe's "The Crystal Locket" in 1935. She wrote and illustrated her own children's books in the 1930s and' 40s.