IntroductionDespite being an important period for the development of movement behaviours (physical activity, sedentary behaviour and sleep), few interventions commencing prior to preschool have been trialled. The primary aim of this trial is to assess the 12-month efficacy of the Let’s Grow mHealth intervention, designed to improve the composition of movement behaviours in children from 2 years of age. Let’s Grow is novel in considering composition of movement behaviours as the primary outcome, using non-linear dynamical approaches for intervention delivery, and incorporating planning for real-world implementation and scale-up from its inception.Methods and analysisA randomised controlled trial will test the effects of the 12-month parental support mHealth intervention, Let’s Grow, compared with a control group that will receive usual care plus electronic newsletters on unrelated topics for cohort retention. Let’s Grow will be delivered via a purpose-designed mobile web application with linked SMS notifications. Intervention content includes general and movement-behaviour specific parenting advice and incorporates established behaviour change techniques. Intervention adherence will be monitored by app usage data. Data will be collected from participants using 24-hour monitoring of movement behaviours and parent report at baseline (T₀), mid-intervention (T₁; 6 months post baseline), at intervention conclusion (T₂; 12 months post baseline) and 1-year post intervention (T₃; 2 years post baseline). The trial aims to recruit 1100 families from across Australia during 2021. In addition to assessment of efficacy, an economic evaluation and prospective scalability evaluation will be conducted.Ethics and disseminationThe study was approved by the Deakin University Human Ethics Committee (2020-077). Study findings will be disseminated through publication in peer-reviewed journals, presentation at scientific and professional conferences, and via social and traditional media.Trial registration numberACTRN12620001280998; U1111-1252-0599.