BACKGROUND: Autism and ADHD are heritable, co-occurrent, and associated with difficulties with executive functioning (cognitive and self-regulation skills which enable us to set and work toward goals). Executive function difficulties, and their negative impacts across cognitive, health and social domains, extend to individuals with first-degree relatives who are autistic or have ADHD, even if they do not meet thresholds for a clinical diagnosis themselves. Supporting executive function development in children with elevated autism traits, or a first-degree relative with autism or ADHD, addresses community priorities for early support to help achieve the best mental health, education and life outcomes. METHODS: This study will evaluate the feasibility and acceptability of a randomized controlled trial (RCT) of a parent-toddler programme entitled "Supporting Toddlers with a connection to autism or ADHD to develop strong Attention, Regulation and Thinking skills" (START). START is a neurodiversity-affirming programme, co-refined through extensive Patient and Public Involvement. Sixty parent-child dyads, in Oxford or Southampton (UK), will be randomized using Sealed Envelope by a researcher not involved in recruitment, delivery or outcome data collection to receive START or usual practice, on a 1:1 ratio. Children (20 months old) will be assessed using questionnaires completed by the parent (not blind to allocation) post-intervention (within 2 weeks of the end of the active intervention wave, when children are aged 27-31 months), and using parent questionnaires and a battery of executive function measures administered by researchers blind to allocation at baseline and follow-up (36 months old). START will be delivered in small groups to 30 parent-child dyads, in community settings. DISCUSSION: We will assess the feasibility of recruiting eligible participants to the study, the reliability of measures of implementation fidelity and degree of implementation fidelity achieved, the appropriateness of proposed outcome and mechanism measures, the acceptability of an RCT of the programme, parental adherence to the programme, logistics of programme delivery, and the acceptability of START, using mixed-method measures of engagement and satisfaction. Results will inform the design and implementation of a definitive RCT of START, and yield broader insights into the delivery and evaluation of complex early-years interventions in community settings. TRIAL REGISTRATION: ISRCTN registry ISRCTN99820028 https://doi.org/10.1186/ISRCTN99820028 .