The Minimising Young Children's Anxiety through Schools (MY-CATS) trial is being conducted to determine whether an online evidence-based parent-guided cognitive behavioural therapy intervention in addition to usual school practice is effective and cost-effective compared with usual school practice in reducing anxiety disorders in children aged 4-7 deemed 'at risk' of anxiety disorders. This update article describes the detailed statistical analysis plan for the MY-CATS trial and reports a review of the underpinning sample size assumptions. The MY-CATS study is a two-arm, definitive superiority pragmatic parallel group cluster randomised controlled trial in which schools will be randomised 1:1 to receive either the intervention (in addition to usual school practice) or the usual school practice only. This update to the (published) protocol provides a detailed description of the study methods, the statistical principles, the trial population and the planned statistical analyses, including additional analyses comprising instrumental variable regression and mediation analysis. ISRCTN82398107 . Prospectively registered on 14 January 2021.