The COVID-19 pandemic required in-person interventions to be adapted for remote delivery all over the globe. In South Africa, an in-person cascaded task-sharing naturalistic developmental behavioural intervention was adapted for telehealth delivery in a low-resource context. Here we describe the adaptations made (the 'what') and reasons for adaptations (the 'why'). The Framework for Modification and Adaptations (FRAME) was used to document the 'what', and the Exploration, Preparation, Implementation, Sustainment (EPIS) framework to describe the 'why'. Systematic member-checking ensured robustness of results. The 'what' included 10 adaptations: selecting WhatsApp as delivery platform, developing images with simple text to communicate intervention concepts, modifying session structure for hybrid delivery, including a caregiver self-reflection checklist, utilizing online practitioner training, supervision, assessment and consent procedures, developing session recording procedures, distributing session materials electronically, and developing caregiver-child interaction recording and uploading protocols. The 'why' included three outer contextual factors (the digital divide, WhatsApp security/privacy policy, and COVID-19 restrictions), three inner contextual factors (characteristics of caregivers and practitioners, ethics board guidance, and school leadership and organizational characteristics) and one innovation factor (support from intervention co-developers). Adaptations were made in response to unchangeable outer contextual factors and through identification of malleable inner contextual factors.