This study aimed to assess the agreement in physical activity (PA) estimates from Garmin Vivofit Jr. 3 (VFJ 3) and Fitbit Ace 3 (Ace 3) with a research-grade accelerometer (wGT3X-BT) in children under free-living conditions. Twenty-five children (Girls: 56%, Age: 10.1 ± 2.5 years, BMI: 17.1 ± 2.4 kg/m2) performed daily activities for 7 consecutive days while wearing VFJ 3, Ace 3, and wGT3X-BT. Pearson correlations, Bland-Altman plots, mean percent error (MPE), mean absolute percent error (MAPE), and equivalence tests were conducted to evaluate the agreement of VFJ 3 and Ace 3 with wGT3X-BT. VFJ 3 and Ace 3 had strong positive correlations (range: r = 0.71 to 0.95) with wGT3X-BT in estimating steps and moderate-to-vigorous PA (MVPA) and provided relatively valid estimates for daily steps. Although Ace 3 had no systematic bias and a low group measurement error (MPE: -10.1%) in estimating MVPA, VFJ 3 consistently overestimated daily MVPA. Collectively, VFJ 3 and Ace 3 can be valid devices to monitor children's PA levels with daily step counts.