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 is 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. Two phases of cognitive pretesting were completed to gather data about how children read, interpret, and answer self-report items. Cognitive pretesting procedures identified validity problems with numerous items leading to modifications. Cognitive pretesting framed by an argument-based approach to validity holds significant potential to improve the developmental validity of child self-report instruments. (Contains 1 figure and 1 table.)