While child development is well-recognized as a complex process which could hardly be decomposed into separate lines or domains in contemporary psychological theories, the decomposition approach is widely used in empirical studies. Based on the cultural-historical theory, this study argues for adopting the unit of analysis as a way to bridge this gap, and finds "role adjustment" as a suitable unit to analyse child development during the transition to school. The case of a 5-year-old second-generation Chinese Australian child's transition to school (with a total of 34 h of observation and 5 h of interview) is presented to verify its feasibility and to illustrate how it could be achieved in a research practice. The case study demonstrates that a child's role adjustment is in essence a process of transforming his/her role system, during which his/her major roles, the relation of the major roles, as well as his/her role-taking patterns change as a whole, and by tracing a child's role adjustment trajectory, how the child is learning and developing, what difficulties he/she encounter, and why these difficulties appear become clear.