Young children's activity and sedentary time were simultaneously measured via the Actical method (i.e., Actical accelerometer and Pfeiffer et al.'s cut-points) and the ActiGraph method (i.e., ActiGraph accelerometer and Pate et al.'s cut-points) at both 15s and 60s epochs to explore possible differences between these two measurement approaches. For seven consecutive days, participants (n = 23) wore both the Actical and ActiGraph side-by-side on an elastic neoprene belt. Device-specific cut-points were applied. Paired sample t-tests were conducted to determine the differences in participants' daily average activity levels and sedentary time (mins/hr) measured by the two devices at 15s and 60s time sampling intervals. Bland-Altman plots were used to examine agreement between Actical and ActiGraph accelerometers. Regardless of epoch length, Actical accelerometers reported significantly higher rates of sedentary time (15s: 42.7 mins/hr vs. 33.5 mins/hr; 60s: 39.4 mins/hr vs. 27.1 mins/hr). ActiGraph accelerometers captured significantly higher rates of moderate-to-vigorous physical activity (15s: 9.2 mins/hr vs. 2.6 mins/hr; 60s: 8.0 mins/hr vs. 1.27 mins/hr) and total physical activity (15s: 31.7 mins/hr vs. 22.3 mins/hr; 60s: = 39.4 mins/hr vs. 25.2 mins/hr) in comparison to Actical accelerometers. These results highlight the present accelerometry-related issues with interpretation of datasets derived from different monitors.