Background Optimal child-rearing practices can help mitigate the consequences of detrimental social determinants of health in early childhood. Given the ubiquity of personal digital technologies worldwide, the direct delivery of evidence-based information about early childhood development holds great promise. However, to make the content of these novel systems effective, it is crucial to incorporate place-based cultural beliefs, traditions, circumstances, and value systems of end users. Objective This paper describes the iterative approach used to develop the Thrive by Five child-rearing app in collaboration with Afghan parents, caregivers (eg, grandparents, aunts, and nannies), and subject matter experts (SMEs). We outline how co-design methodologies informed the development and cultural contextualization of content to meet the specific needs of Afghan parents and the content was tested and refined in collaboration with key Afghan stakeholders. Methods The preliminary content was developed based on a comprehensive literature review of the historical and sociocultural contexts in Afghanistan, including factors that influence child-rearing practices and early childhood development. After an initial review and refinement based on feedback from SMEs, this content was populated into a beta app for testing. Overall, 8 co-design workshops were conducted in July and August 2021 and February 2022 with 39 Afghan parents and caregivers and 6 SMEs to collect their feedback on the app and its content. The workshops were audio recorded and transcribed; detailed field notes were taken by 2 scribes. A theoretical thematic analysis using semantic codes was conducted to inform the refinement of existing content and development of new content to fulfill the needs identified by participants. Results The following 4 primary themes were identified: child-rearing in the Afghan sociocultural context, safety concerns, emotion and behavior management, and physical health and nutrition. Overall, participants agreed that the app had the potential to deliver valuable information to Afghan parents; however, owing to the volatility in the country, participants recommended including more activities that could be safely done indoors, as mothers and children are required to spend most of their time at home. Additionally, restrictions on public engagement in music required the removal of activities referencing singing that might be performed outside the home. Further, activities to help parents reduce their children’s screen time, promote empathy, manage emotions, regulate behavior, and improve physical health and nutrition were requested. Conclusions Direct engagement with Afghan parents, caregivers, and SMEs through co-design workshops enabled the development and refinement of evidence-based, localized, and contextually relevant child-rearing activities promoting healthy social, emotional, and cognitive development during the first 5 years of children’s lives. Importantly, the content was adapted for the ongoing conflict in Afghanistan with the aim of empowering Afghan parents and caregivers to support their children’s developmental potential despite the security concerns and situational stressors.