Accelerometers could potentially be used to assess physical activity and sleep using the same monitor; however, two different positions are typically used for the assessment of physical activity and sleep (waist and wrist, respectively). The aim of this study is to evaluate the concordance between wrist- and waist-worn Actigraphs for sleep scoring using existing algorithms developed for placement on the wrist. Sixty-two Danish children aged 10.3 ± 0.6 years (mean ± standard deviation) wore an accelerometer (Actigraph GT3X+ Tri-Axis Accelerometer Monitor) on the right hip and on the non-dominant wrist for a continuous 7 days and 8 nights. The minute-by-minute sleep–wake agreement was evaluated and the concordance among sleep indicators was assessed using one-way ANOVA, Bland–Altman plots and Pearson correlations. The sensitivity, specificity and accuracy were 98.8–99.7, 29.8–46.9 and 86.6–88.9%, respectively. Total sleep time and sleep efficiency were higher when assessed from the waist compared to the wrist measurement site (73.8 min, P < 0.001; 12.6%, P < 0.001, respectively). In conclusion, the waist-worn and wrist-worn Actigraph monitors cannot be used interchangeably for the measurement of sleep indicators in children using the present algorithms. However, the waist-worn Actigraph monitor can provide a proxy measure of total sleep time for ranking purposes in epidemiologic studies.