Complex sentence: As her career progressed she shifted her focus primarily to writing: she wrote and illustrated children's books in the 1930s and' 40s, though after 1951 her children's books were illustrated by other artists.
Split into simpler sentences: As her career progressed, she shifted her focus primarily to writing. She wrote and illustrated her own children's books in the 1930s and' 40s, though after 1951 her children's books were illustrated by other artists.