Background: There are limited evidence-based programs for children living in Queensland, Australia, who are at risk of overweight or obesity. Despite the known importance of prevention initiatives, an online, locally relevant program supporting sustainable health behaviors does not exist. This study aimed to understand the perspectives of parents/guardians and health professionals regarding important aspects of an online childhood obesity prevention program. Methods: This pragmatic, mixed-methods study was conducted from March to December 2020. Recruitment included participants from two groups involved with children aged 2-17 years, health professionals, and parents/guardians. Phase 1 involved dissemination of an online survey. Questions addressed program structure, content delivery (including nutrition, physical activity, and parenting practices), program evaluation, and information dissemination. Descriptive statistics were used to describe survey data to inform the delivery of Phase 2, where two focus groups further explored the topics. Thematic analysis was used to investigate the qualitative data. Results: Twenty-eight health professionals and 11 parents/guardians completed the survey, and 14 health professionals and 6 parents/guardians participated in the focus groups. Participants believed that the most beneficial approach would target younger children with family-based interventions, via a nontraditional structure. There was a strong preference for interactive content, gamification, and practical resources to translate knowledge into practice. Parents emphasized that there should be no assumption of basic knowledge, and that decision fatigue is a barrier to engagement. Conclusion: Participants provided clear direction regarding key aspects for future development of an online prevention program, highlighting the importance of codesign to tailor the program to identified needs.