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Summary: Background: Population‐specific accelerometer cut‐points are required to accurately determine the accumulation of physical activity of various intensities. Objectives: A calibration study was conducted (i) to determine the cut‐points for the ActiGraph GT3X+, non‐dominant, wrist‐mounted accelerometer in children aged 8–12 years and (ii) to compare classification accuracies among the accelerometer's three axes and vector magnitude (VM) values. Methods: Forty‐five children aged 8–12 years performed up to seven activities while wearing accelerometers on their non‐dominant wrist. Activities were performed in a summer day camp setting, represented free‐living activities, and lasted for 10 min with minutes 5–8.5 used for analysis. Direct observation and percentage of heart rate reserve were used to determine activity intensity. Results: Receiver operator characteristic (ROC) analyses resulted in area under the curve values of all three axes and VM ranging 0.82–0.89, 0.80–0.83, 0.62–0.67 and 0.86–0.89 for light, moderate, vigorous and moderate‐to‐vigorous activity intensities. Additionally, regression analyses resulted in prediction equations with R2 values ranging from 0.70 to 0.77. Conclusion: Results found comparable activity intensity classification accuracies from the ActiGraph GT3X+ wrist‐worn accelerometer to previously published studies. Based on ROC and regression analyses, activity intensities can be distilled from this accelerometer using axis 1, axis 2 or VM values with similar classification accuracy. [ABSTRACT FROM AUTHOR]