Piaget is one of the principle developmental psychologists who has investigated cognitive development over the long run. Vygotsky and others have looked at if from a narrower perspective. Do not forget to include them as appropriate.
A subsection devoted to language development would be appropriate.
Identify the range of ages associated with this stage.
What stage(s) of cognitive development does Piaget associate with this period of life?
What can the child do now that he or she could not do before?
How do we know what the child can do? (What, if any, special methodological procedures do we employ to find out?)
Be sure to use, and define or illustrate, the proper terms used by developmental psychologists.
I would expect this assignment to be at least a page or two in length.
Provide proper references.