Growing concern about the global burden of child mental health disorders has generated an increased interest in population-level efforts to improve child mental health. This in turn has led to a shift in emphasis away from treatment of established disorders and towards prevention and promotion. Prevention efforts are able to draw on a substantial epidemiological literature describing the prevalence and determinants of child mental health disorders. However, there is a striking lack of clearly conceptualised and measurable positive outcomes for child mental health, which may result in missed opportunities to identify optimal policy and intervention strategies. In this paper, we propose an epidemiological approach to child mental health which is in keeping with public health principles and with the WHO definition of health, and which is grounded in current thinking about child development. Constructs such as competence offer the opportunity to develop rigorous outcome measures for epidemiological research, while broader ideas about 'the good life' and 'the good society' derived from philosophical thinking can enable us to shape policy initiatives based on normative ideas of optimal child mental health that extend beyond individuals and undoubtedly beyond the traditional boundaries of the health sector.