Aim The Movement Assessment Battery for Children‐2 (MABC‐2) uses age‐grouped scoring, which will result in relative motor functioning being overestimated for some children and underestimated for others. In this paper, we measure these errors and discuss their consequences. Method We pool data from two validation studies to obtain a sample of 278 children assessed with the MABC‐2 (mean (SD) age: 5 years, 0 months (9.6 months); 142 female). We used regression to measure the association between standard score and relative age, and used these results to estimate misclassification rates at the MABC‐2's recommended thresholds. Results Movement Assessment Battery for Children‐2 scores were distributed as expected (mean (SD) = 10.4 (2.8)). We estimated that the standard score varied by 2.76 units (0.92 SDs) per year of relative age. Depending on threshold and age bandwidth, this implies overall misclassification rates from 9% to 23%. Interpretation Relative age differences in MABC‐2 scores led to substantial systematic error for young children. These errors can affect MABC‐2 validity, longitudinal stability and agreement with other tools, which may reduce the appropriateness of care offered to children. Scoring approaches that may reduce or eliminate these errors are outlined.