Background: Anxiety disorders are common among primary-school aged children, but few affected children receive evidence-based treatment. Identifying and supporting children who experience anxiety problems through schools would address substantial treatment access barriers that families and school staff often face. We have worked with families and school staff to co-design procedures that incorporate screening, feedback for parents, and the offer of a brief intervention in primary schools. This study sets out to assess the feasibility of a subsequent school-based cluster randomised controlled trial to evaluate these procedures. Our objectives are to ensure our procedures for identifying and supporting children with anxiety difficulties through primary schools are acceptable and there are no negative impacts, to estimate recruitment and retention rates, and to identify any changes needed to study procedures or measures. Methods: We will recruit six primary/junior schools in England (2 classes per school), and invite all children (aged 8–9) (n = 360) and their parent/carer and class teacher in participating classes to take part. Children, parents and class teachers will complete questionnaires at baseline and 12-week follow-up. Children who ‘screen positive’ on a 2-item parent-report child anxiety screen at baseline will be the target population (expected n = 43). Parents receive feedback on screening questionnaire responses, and where the child screens positive the family is offered support (OSI: Online Support and Intervention for child anxiety). OSI is a brief, parent-led online intervention, supported by short telephone sessions with a Children’s Wellbeing Practitioner. Participants’ experiences of study procedures will be assessed through qualitative interviews/discussion groups. Discussion: Evidence-based procedures for identifying and supporting children with anxiety difficulties through primary schools would improve children’s access to timely, effective intervention for anxiety difficulties. Trial registration: ISRCTN registry: ISRCTN30032471. Retrospectively registered on 18 May 2021.