AIMS: The Patient-Reported Outcomes Measurement Information System (PROMIS) Mobility score is increasingly used as an outcome tool in lower limb fractures in children. We sought to determine the minimal clinically important difference (MCID) in these patients, using an anchor-based approach. METHODS: A prospective longitudinal cohort study involved the use of the PROMIS questionnaires alongside an anchor question at five timepoints over a period of six months. Children aged between five and 15 years with a newly diagnosed, isolated fracture of the femur, tibia, fibula, ankle, or metatarsals were included. Either the proxy or self-reported version of the PROMIS tool was used, depending upon the age of the child. RESULTS: A total of 150 children were enrolled, with a broad range of lower limb fractures. A steady improvement in the PROMIS was seen among all children during follow-up, with near-universal improvement at each timepoint. The PROMIS score representing 'Slightly better' function (i.e. the commonly accepted definition of MCID) was 2.0, which triangulated well with the distribution method for a 'small' effect size. However, the context and type of intervention need to be considered when choosing an effect size for a clinical study. CONCLUSION: These findings will inform the choice of effect sizes in future paediatric lower limb trauma trials, and provide clinicians with an understanding of how to interpret PROMIS scores in terms of a patient's recovery and the effectiveness of treatment.