Depression and anxiety are common mental health problems among adolescents worldwide. Extant research has found that intelligence, emotion, and failure-is-debilitating beliefs (fixed mindsets) are closely related to more depression and anxiety symptoms, hopelessness, and suicidality. Recent research also points to the importance of parental mindset, which can strongly influence children's affect, behavior, and mental health. However, the effects of parent-child mindset interventions on a child's internalizing problems have not yet been empirically examined. As recent evidence has shown the promise of single-session interventions in reducing and preventing youth internalizing problems, this study develops and examines a parent and child single-session intervention on mindsets of intelligence, failure, and emotion (PC-SMILE) to tackle depression and anxiety in young people. Using a 3-arm randomized controlled trial, this study will examine the effectiveness of PC-SMILE in reducing depression and anxiety symptoms among children. We hypothesize that compared to the waitlist control group, the PC-SMILE group and child single-session intervention on mindsets of intelligence, failure, and emotion (C-SMILE) group will significantly improve child depression and anxiety (primary outcome) and significantly improve secondary outcomes, including children's academic self-efficacy, hopelessness, psychological well-being, and parent-child interactions and relationships, and the PC-SMILE is more effective than the C-SMILE. A total of 549 parent-child dyads will be recruited from 8 secondary schools and randomly assigned to either the PC-SMILE intervention group, the C-SMILE intervention group, or the no-intervention waitlist control group. The 45-minute interventions include parent-version and child-version. Both parents and students in the PC-SMILE group receive the intervention. Students in C-SMILE group receive intervention and their parents will receive intervention after all follow-up ends. Students in 3 groups will be assessed at 3 time points, baseline before intervention, 2 weeks post intervention, and 3 months post intervention, and parents will be assessed in baseline and 3-month follow-up. The intention-to-treat principle and linear-regression-based maximum likelihood multilevel models will be used for data analysis. Recruitment started in September 2023. The first cohort of data collection is expected to begin in May 2024 and the second cohort will begin in September 2024. The final wave of data is expected to be collected by the end of the first quarter of 2025. The results are expected to demonstrate improved anxiety and depression among students assigned to the intervention condition, as well as the secondary outcomes compared to those in the control group. The efficacy and effectiveness of the intervention will be discussed. This study is the first attempt to develop a web-based single-session intervention for students and their parents to enhance their well-being in Hong Kong and beyond, which potentially contributes to providing evidence-based recommendations for the implementation of brief digital parent-child interventions. PRR1-10.2196/63220.