Sigmund Freud

Stressed importance of childhood events and experiences, describes child development as a series of 'psychosexual stages.' In "Three Essays on Sexuality" (1915), Freud outlined these stages as oral, anal, phallic, latency and genital. Each stage involves the satisfaction of a libidinal desire and can later play a role in adult personality. Freud believed that the way parents dealt with children's basic sexual and aggressive desires would determine how their personalities developed and whether or not they would end up well-adjusted as adults. Freud thought that all babies are initially dominated by unconscious, instinctual and selfish urges for immediate gratification which he labeled the Id. As babies attempt and fail to get all their whims met, they develop a more realistic appreciation of what is realistic and possible, which Freud called the "Ego". Over time, babies also learn about and come to internalize and represent their parents' values and rules. These internalized rules, which he called the "Super-Ego", are the basis for the the developing child's conscience that struggles with the concepts of right and wrong and works with the Ego to control the immediate gratification urges of the Id.

Erik Erikson

Erikson believed that throughout the human lifespan each stage of development was focused on overcoming a conflict. He believed that children play an active roll in gaining knowledge of the world. 

Jean Piaget

Jean Piaget spent much of his professional life listening to children, watching children and poring over reports of researchers around the world who were doing the same. Through his research, he found that children don't think like grownups. After thousands of interactions with young people often barely old enough to talk, Piaget began to suspect that behind their cute and seemingly illogical utterances were thought processes that had their own kind of order and their own special logic. Einstein called it a discovery "so simple that only a genius could have thought of it." Piaget's insight opened a new window into the inner workings of the mind. In his books, Piaget defines several stages through which a child passes, each new stage representing an improvement in reasoning. He provides examples of experiments and questions applied to different age groups that clearly display the changes, and describes the implications. 
Four development stages:
  • Sensorimotor stage: from birth to age 2 years (children experience the world through movement and senses and learn object permanence) child begins with adaption and grasping knowledge
  • Preoperational stage: from ages 2 to 7 (acquisition of motor skills)
  • Concrete operational stage: from ages 7 to 11 (children begin to think logically about concrete events)
  • Formal operational stage: after age 11 (development of abstract reasoning)