BackgroundMost children with developmental disabilities (DD) live in low- and middle-income countries, but access to services is limited, impacting their ability to thrive. Pilot study findings of the World Health Organization’s Caregiver Skills Training (WHO CST) intervention, which equips caregivers with strategies to facilitate learning and adaptive behaviours in children with DD, are promising but evidence from an appropriately powered trial delivered by non-specialist facilitators is lacking. This study will investigate the effectiveness and the resource impacts and costs and consequences of the WHO CST intervention in four sites in rural and urban Kenya and Ethiopia.MethodsThis is a 2-arm multi-site hybrid type-1 effectiveness implementation cluster randomised controlled superiority trial. After baseline assessments (T0) are completed by participants in clusters comprising 7 to 10 caregiver-child dyads, the clusters will be randomised to either the WHO CST intervention arm or a waitlist enhanced care as usual control arm. Further assessments will be completed at endpoint (T1, 18 ± 2 weeks after randomisation) and follow-up (T2, 44 ± 2 weeks after randomisation). The intervention comprises three individualised home visits and nine group sessions with trained non-specialist facilitators. Participants in the control arm will receive the intervention after completing follow-up assessments. We aim to recruit 544 child-caregiver dyads, evenly distributed across the two arms and countries. The co-primary outcomes are the child-focused Child Behavior Checklist (assessing emotional and behavioural problems) and the caregiver-focused Pediatric Quality of Life Inventory (assessing caregiver quality of life), both assessed at endpoint. Secondary outcome measures comprise the two co-primary outcomes at follow-up and ten additional outcome measures at endpoint, assessing stigma-based experiences, depressive symptoms, household food insecurity, child disciplinary strategies and beliefs, CST knowledge and skill competencies, caregiver and child quality of life, social support, and children’s communication modes and functions. After quantitative follow-up assessments are completed, a mixed-methods evaluation approach will be used to investigate implementation processes and acceptability, feasibility, and potential sustainability of the intervention.DiscussionThe study’s findings will provide evidence of the effectiveness and resource impacts and costs and consequences of a non-specialist-delivered intervention in under-resourced contexts in one low-income and one middle-income country in East Africa. Findings will inform future research, intervention, and policy efforts to support children with DD and their families in under-resourced majority world contexts.Trial registrationPan African Clinical Trial Registry PACTR202310908063134. Registered on October 16, 2023.