There is a lack of research evaluating the effects of digital textbooks and digital apps on learning for early elementary school learners. Therefore, this study conducted a randomized controlled trial to evaluate how an iPad application for learning English as a foreign language impacted children's grammar understanding, satisfaction, persistence, and confidence compared to a digital textbook. A total of 119 Japanese first- and second-graders participated in the study. They were randomly divided into groups studying English using "digital books" or "applications" over five days. Pre- and post-intervention tests and questionnaires were used to measure their understanding of the study content, satisfaction, persistence, and confidence in English. Application-based learning was more effective than digital books in terms of grammar comprehension and was significantly better for satisfaction and persistence. The results suggested the use of specific criteria for selecting multimedia learning materials, including a high degree of learner control, autonomy to manipulate the learning environment, feedback on learning activities, and guaranteed interactions with the learning materials.