OBJECTIVE: In the context of the importance of valid self-report measures to research and evidence-based practice in social work, an argument-based approach to validity is presented and the concept of developmental validity introduced. Cognitive development theories are applied to the self-report process of children and cognitive pretesting is reviewed as a methodology to advance the validity of self-report instruments for children. An application of cognitive pretesting is presented in the development of the Elementary School Success Profile. METHOD: Two phases of cognitive pretesting were completed to gather data about how children read, interpret and answer self-report items. RESULTS: Cognitive pretesting procedures identified validity problems with numerous items leading to modifications. CONCLUSIONS: Cognitive pretesting framed by an argument-based approach to validity holds significant potential to improve the developmental validity of child self-report instruments.