This study used eye-tracking to examine whether extraneous illustration details—a common design in beginning reader storybooks—promote attentional competition and hinder learning. The study used a within-subject design with first- and second-grade children. Children ( n  = 60) read a story in a commercially available Standard condition and in a Streamlined condition, in which extraneous illustrations were removed while an eye-tracker recorded children’s gaze shifts away from the text, fixations to extraneous illustrations, and fixations to relevant illustrations. Extraneous illustrations promoted attentional competition and hindered reading comprehension: children made more gaze shifts away from text in the Standard compared to the Streamlined condition, and reading comprehension was significantly higher in the Streamlined condition compared to the Standard condition. Importantly, fixations toward extraneous details accounted for the unique variance in reading comprehension controlling for reading proficiency and attending to relevant illustrations. Furthermore, a follow-up control experiment ( n  = 60) revealed that these effects did not solely stem from enhanced text saliency in the Streamlined condition and reproduced the finding of a negative relationship between fixations to extraneous details and reading comprehension. This study provides evidence that the design of reading materials can be optimized to promote literacy development in young children.