Theorists

John Bowlby -

John Bowlby was a child psychiatrist who researched into the effects on children of separation from their mothers.
He is well known for his theory of attachment that is based on the premise that the mother-baby attachment is unique and different from any other relationship the child may have. Bowlby emphasized the importance of the bond between infants and their primary carers,usually their mothers. Bowlby claimed that individuals future mental health depended on successful attachment in the first few months of life.
Bowlbys theory included the effect of separation on infants when they were separated from their attachment figures, infants become distressed and anxious.
When separation is prolonged in the first two to five years of a childs life, the child will experience loss and grief. The child will initially cry and protest. As the period of separation continues the child despairs and withdraws, eventually becoming detached from relationships with people, here is some examples of what he believed ; theory of attachment, separation, grief and loss, importance of primary bond between mother and baby, need to form attachments is biological, first 5 years of life most important for later personality development, children without warm, continuous relationship with mother suffer maternal deprivation. 

Jean Piaget's -


Jean Piaget's theory of cognitive development suggests that children move through four different stages of mental development these stages are, sensory motor stage (birth to 18months/2years) , per-operational stage (2 to 6/7years), the concrete operational stage (7 to 11years) and formal stage (11years to 18+). Piaget's ideas included concepts such as symbolism, conservation, ego centrism and object permanence.
Object permanence is the understanding that objects continue to exist even when they cannot be seen, heard, or touched, for example is a child see's a toy and you take that toy away from them and hide is behind your back that child will still know that,that toy still exists they just can't touch it.

His theory focuses on understanding how children acquire knowledge and the understanding of nature intelligance. Piaget beleived our senses take in information about things around us and our brains store this which affect our behavior. He also believed that cognitive development occurs as a result of active exploration and discovery of the world by children, he carried out some experiments on the children for example 'The 3 mountains task' where he showed 3 mountains to a few of the children but sat them at different angles but some of the children were unable to see the mountains from a different perspective to highlight children are ego centric.
He also beleived that schemata are patterns of behaviour which we use to guide and direct our behavior. For example, the baby flaps out her hand and accidentally reaches toys. Eventually, because of this kind of accidental exploartion, the baby is able to control movements in order to reach the toy. The movement (or operation) is based on mental schema that has developed for this action and that schemata is developed through assimilation and accommodation.

Assimilation is the way in which a child takes the important elements of their experience.
Accommodation is the process that involves fitting what the child has learned from new experiences.