Children at play.

Children often enforce gender roles quite rigidly. Perhaps we can draw a parallel with the development of play. During the sensorimotor period, play tends to be solitary and self-absorbed, as the child develops its ability to manipulate the physical world. With a group of other children, it will tend to continue to play alone, but look on at others. In time co-operative play develops, with highly organised group games.

However, the rules are rigid. If there are any disagreements, the group is likely to break up. It is not until a later stage that rules become negotiable. Perhaps, then, the rules of gender behaviour are seen as rigidly either/or, in terms of the rules provided by the cultural milieu in which the group of children live. It is not until a later developmental stage that gender behaviour can be viewed as flexible, a stage that many adults and cultures have not reached.

Citation:
Bland. J. (1998) About Gender: Children at Play
http://www.gender.org.uk/about/02psycho/23_comnt.htm