Children with dyslexia face a documented working memory deficit that makes them uniquely vulnerable to the extraneous cognitive load generated by visually complex illustrated learning materials — yet no study has empirically identified the precise level of visual complexity at which this load exceeds their cognitive capacity and begins to impair comprehension and recall. This paper addresses that gap. Drawing on Cognitive Load Theory, the Cognitive Theory of Multimedia Learning, and Dual Coding Theory, and grounded in a systematic review of the dyslexia, multimedia learning, and visual design literatures, the paper proposes a rigorous experimental methodology to determine this threshold for the first time. The proposed study exposes children with dyslexia and typically developing peers aged 7–10 years to four systematically graduated levels of visual complexity — low, moderate, high, and excessive — across a single, purpose-designed illustrated narrative, and measures comprehension accuracy, recall depth, response time, self-reported cognitive difficulty, and behavioural engagement. A within-subjects, between-groups mixed design with breakpoint regression analysis is used to identify the specific complexity level at which cognitive load threshold is exceeded for each group, and moderated regression analyses examine whether working memory capacity predicts threshold location within the dyslexia group. The paper introduces the concept of a population-specific visual complexity threshold as a theoretically novel extension of Cognitive Load Theory, contributes a validated multi-indicator convergence method for threshold identification in child populations, and offers the first empirically grounded illustration complexity guidelines for the inclusive design of educational picture books and digital learning materials for children with dyslexia.