.  **Competing interests:** None.  **Ethics approval:** Health Research and Development Committee, Ministry of Health in Botswana; Research and Ethics Committee, Ministry of Health and Social Welfare in Lesotho; the National Health Sciences Research Committee, Ministry of Health in Malawi; the Comité Nacional de Bioética para a Saude, Ministerio da Saude in Mozambique; the Research Management Committee, Ministry of Health and Social Services in Namibia; the CIET Trust Research Ethics Committee in South Africa; the Scientific and Ethics Committee, Ministry of Health and Social Welfare in Swaziland; the Institutional Review Board, Ifakara Health Research and Development Centre in Tanzania; the Permanent Secretary, Ministry of Health, Zambia and the Medical Research Council of Zimbabwe. In each country, we also received written authority from the Ministry of Education to interview children in school.  **Provenance and peer review:** Not commissioned; externally peer reviewed.  **Data sharing statement:** 