"what is the primary psychosocial milestone of childhood? "

Positive development of self-concept

Self-awareness or self-concept is a knowledge or awareness about oneself including beliefs about ones abilities, values, goals, roles, personality traits, and physical characteristics. 

For an infant, the first signs of the development of self-concept is self-recognition. A common test performed is the mirror test, where a red dot is drawn somewhere on the infant's face (e.g. forehead or nose).

Upon looking at himself/herself in the mirror, if the infant touches or attempts to wipe off the dot on his/her face and not of his/her reflection in the mirror, then the infant has already developed self-awareness (i.e. self-recognition). By 18 months of age, 50% of infants are able to recognize themselves using the mirror test and by 24 months, this percentage increases to 65%.