Introduction Digital mental health interventions (DMHIs) offer a promising alternative or adjunct treatment method to face-to-face treatment, overcoming barriers associated with stigma, access, and cost. This project is embedded in user experience and co-design to enhance the potential acceptability, usability and integration of digital platforms into youth mental health services. Objective To co-design a digital mental health platform that provides self-directed, tailored, and modularised treatment for young people aged 7–17 years experiencing anxiety, depression and other related problems. Methods Sixty-eight participants, aged 7–17 years, engaged in one of 20 co-design workshops. Eight workshops involved children (n  =  26, m  =  9.42 years, sd  =  1.27) and 12 involved adolescents (n  =  42, m  =  14.57 years, sd  =  1.89). Participants engaged in a variety of co-design activities (e.g., designing a website home page and rating self-report assessment features). Workshop transcripts and artefacts (e.g., participants’ drawings) were thematically analysed using Gale et al.'s Framework Method in NVivo. Results Six themes were identified: Interactive; Relatable; Customisable; Intuitive; Inclusive; and Personalised, transparent and trustworthy content. The analysis revealed differences between children's and adolescents’ designs and ideas, supporting the need for two different versions of the platform, with age-appropriate activities, features, terminology, and content. Conclusions This research showcased co-design as a powerful tool to facilitate collaboration with young people in designing DMHIs. Two sets of recommendations were produced: 1) recommendations for the design, functionality, and content of youth DMHIs, supported by child- and adolescent-designed strategies; and 2) recommendations for clinicians and researchers planning to conduct co-design and intervention development research with children and adolescents.