Introduction: Digital mental health interventions (DMHIs) can offer affordable, accessible and anonymous delivery of mental health treatment as an alternative or complement to face-to-face services. To enhance acceptability of, and treatment engagement with, youth DMHIs, they should be co-designed with young people and their caregivers. This study focuses on co-design with caregivers. Objective: To explore caregivers’ perspectives, preferences, and ideas to inform the co-design of a digital youth mental health platform for anxiety and depression: ‘Momentum’. Methods and Measures: Six group discussions were conducted with 16 caregivers of young people aged 7–17 years. In relation to Momentum, and DMHIs more broadly, participants’ views were sought on purpose and acceptability, access, assessments and feedback, information sharing, caregiver involvement and engagement. Data were thematically analysed using an inductive codebook approach, guided by template analysis. Results: Seven themes were identified: (1) providing therapeutic and educational resources; (2) promoting shared and positive communication about mental health; (3) enhancing user experience and understanding; (4) facilitating personalisation and offering customisation of platform features; (5) encouraging end-user engagement through interactivity, incentives, relatability and attracting and maintaining attention; (6) enabling caregivers to provide support while promoting young people's independence; and (7) reducing concerns about, and enhancing trust in, DMHIs. Conclusions: Participants expressed favourable views towards DMHIs and Momentum. They shared design ideas for a user-friendly, engaging, interactive, trustworthy, personalised and transparent platform that offered educational resources and mental health treatment. Two sets of recommendations were derived from the study findings: (1) recommendations for co-designing DMHIs with caregivers and (2) recommendations for the design of youth DMHIs.