This article discusses the science-based diagnostic observation in a Dutch girls’ reformatory in the 1950s. Scientisation of the observation implied that to the medical examination upon entry and observation of a child’s behaviour were added a psychological assessment, a psychiatric examination, and an inquiry into the family of origin. Inspired by dynamic psychology it was part of a strategy to promote a more professional and individualised treatment in child protection. The research addresses the claims to an additional value of child science in this procedure. It turns out that the psychological and psychiatric knowledges weighed heavier than the input from non-academic staff in regard to the representation of a girl in the observation report and advice to the juvenile court. Information concerning a girl’s observed behaviour and acceptance in the group was overruled by Freudian beliefs regarding the consequences of early childhood experiences for her unconscious feelings.