BACKGROUND: Vulnerable children, including those with neuro-developmental delays and disabilities, often face barriers in accessing early primary education, thus hindering progress toward Sustainable Development Goal 4.2. Evidence-based interventions are essential to enhancing inclusivity and establishing sustainable implementation strategies to address this challenge. This study, Every Newborn-Reach up Early Education Intervention for All Children (EN-REACH), builds on the previous Every Newborn- Simplified Measurement Integrating Longitudinal Neurodevelopmental and Growth (EN-SMILING) observational cohort study. This paper provides the protocol for a cluster randomized controlled trial (cRCT) to evaluate the effectiveness of a parenting group intervention program for enhancing school readiness in Bangladesh, Nepal, and Tanzania, and an embedded process evaluation to inform scalability and feasibility. METHODS: EN-REACH is a cRCT with at least 150 clusters to evaluate the impact of a parent training program led by trained parent-teacher facilitator pairs, focusing on children aged 4 ~ 6 years preparing for preschool. Approximately 500 participants from the EN-SMILING cohort at each site have been identified. A geographic information system will define ~ 50 clusters in each of the three countries, each with approximately ten parent-child dyads. Half the clusters will be randomly assigned to intervention and control groups. The primary outcome is "school readiness", assessed using the Measuring Early Learning Quality and Outcomes tool. Secondary outcomes include Intelligence Quotient, child functioning, growth, visual, and hearing assessments. Data will be collected at baseline, and post-intervention data following implementation of the parent group intervention sessions over approximately 5 months. Quantitative data on coverage and quality care, combined with qualitative insights from children, caregivers, facilitators, and stakeholders' perspectives, will be used to conduct a process evaluation applying the Reach, Effectiveness, Adoption, Implementation, and Maintenance (RE-AIM) framework.  DISCUSSION: This protocol details a trial focused on enhancing school readiness and cognitive abilities in young children, inclusive of those with disabilities, aiming to bridge gap from home to early primary education. EN-REACH aims to provide insights into the effectiveness and acceptability of a co-designed disability-inclusive school readiness program in three countries, potentially impacting national and global policies for all children, including those with disabilities. TRIAL REGISTRATION: The trial was retrospectively registered on clinicaltrials.gov on 29 February 2024 (NCT06334627).