Traditionally, learning among young students has taken place within structured, physical classroom settings. However, the emergence of distance learning has introduced a diverse range of learning methods, including online, hybrid, and blended approaches. When the COVID-19 pandemic led to extended delays in in-person instruction, use of educational technologies such as asynchronous videos and online platforms were deployed to deliver mathematics curricula aligned with the Common Core State Standards (CCSS), though best practices for teaching mathematics asynchronously are not well studied. This study focuses on exploring the effectiveness of a math course on proportional reasoning that was co-designed, developed, and deployed in 5th and 6th grade Orange County classrooms. Examining the learning experience design (LXD) paradigm, this research focuses on discerning its influence on (n = 303) children's engagement during their involvement in an online, video-based math course. LXD is implemented by combining evidence-based pedagogical instructional design with human-centered user experience (UX) design. The study utilized a structural equation model to analyze the relationships between learners' user experiences, situational interest, mind-wandering, and online engagement. The results demonstrated significant direct effects between students' situational interest, user experience, and their level of online engagement. Findings also indicate that students' situational interest and mind-wandering significantly mediate the relationship between their user experiences and online engagement. These results have important theoretical and practical implications for researchers, designers, and instructors. By combining evidenced-based pedagogical learning design with human-centered user experience design, LX designers can promote situational interest, reduce mind-wandering, and increase engagement in elementary mathematics courses conducted in asynchronous online settings.