Complex sentence: At the age of three her parents moved to Kenya, then British East Africa, where she spent an adventurous childhood playing and hunting with native Africans, and, on her family's farm, developing a knowledge of and affection for horses.
Split into simpler sentences: At the age of three her parents moved to Kenya, which was then British East Africa, where she spent an adventurous childhood learning, playing and hunting with the natives. On her family's farm, she developed a knowledge of and affection for horses.