The majority of children in Nigeria are unable to access mental health services. In this resource-poor setting, a school-based mental health service can be used to reach children who would otherwise not have access. An essential first step in the development of a school-based mental health programme is a needs assessment. Key informants (KIs) from southwest Nigeria were interviewed to identify their perspectives on child mental illness and needs for a school mental health programme. Data were analysed using interpretative phenomenological analysis. Although KIs sometimes used derogatory terms to describe mental illness, they were able to give full descriptions of different kinds of mental illnesses in children and a range of causes based on the bio-psychosocial model of disease. KIs acknowledged deficiencies in their training even though they currently use parent, child and environment-centred interventions to deal with mental health problems in school. KIs reported teachers as comfortable with handling mental health issues in children and suggested interventions that included development of basic and ongoing training. Barriers, such as poverty, ignorance and stigma need to be addressed, while government involvement and enlightenment campaigns are critical components of a successful programme.