Background There are high rates of mental health needs in children in the United Kingdom, and parents are increasingly seeking help for their children's needs. However, there is not enough access to child and adolescent mental health services and parents are seeking alternative forms of support and information, often from web-based sources. Mumsnet is the largest web-based parenting forum in the United Kingdom, which includes user-created discussions regarding child mental health. Objective This qualitative investigation aimed to explore the emergent themes within the narratives of posts regarding child mental health on Mumsnet and to extrapolate these themes to understand the purpose of Mumsnet for parents of children and young people with mental health needs. Methods A total of 50 threads from Mumsnet Talk Child Mental Health were extracted. Following the application of inclusion and exclusion criteria, 41 threads were analyzed thematically using the framework approach, a form of qualitative thematic analysis. Results In total, 28 themes were extracted and organized into 3 domains. These domains were emotional support, emotional expression, and advice and information. The results suggested that parents of children with mental health needs predominantly use Mumsnet to offer and receive emotional support and to suggest general advice, techniques, and resources that could be applied outside of help from professional services. Conclusions This paper discusses the future of health information seeking. Future research is required to establish initiatives in which web-based peer-to-peer support and information can supplement professional services to provide optimum support for parents of children with mental health needs.