This chapter examines how struggling young readers respond to one-to-one diagnostic assessment interventions and describes by offering considerations for designing innovative young learner assessment. Cognitive diagnostic assessment (CDA) is intended to provide individual students with diagnostic feedback customized to their strengths and weaknesses in cognitive processing skills associated with successful performance outcomes. CDA is driven by a growing interest in learning-oriented assessment approaches to meeting pedagogical needs to support learners with detailed diagnostic feedback. Based on multidimensional item response theory (IRT) latent class modeling, CDA provides diagnostic profiles of individual students' mastery status of user-specified skills in detail. The chapter considers learners as dynamic systems within which the interplay between intrapersonal variables and environmental elements mutually influence their cognitive growth necessary for developing bi- and multi-lingual competence. The resulting growth is unlikely to be linear over time and uniform across multiple dynamic systems.