The intra-subject, the inter-operator, and the inter-laboratory variabilities are the main sources of uncertainties in gait analysis, and their effects have been partially described in the literature for adult populations. This study aimed to extend the repeatability and reproducibility analysis to a pediatric population, accounting for the effects induced by the intra-subject variations, the measurement setup, the marker set configuration, and the involved operators in placing markers and EMG electrodes. We evaluated kinematic, kinetic and EMG outputs collected from gait analyses performed on two healthy children in two laboratories, by two operators, and with two marker placement protocols. The two involved centers previously defined a common acquisition procedure based on their routine pipelines. The similarity of kinematic, kinetic, and EMG curves were evaluated by means of the coefficients of the Linear Fit Method, and the Mean Absolute Variability with and without the offset among curves. The inter-operator variability was found to be the main contribution to the overall reproducibility of kinematic and kinetic gait data. On the contrary, the main contribution to the variability of the EMG signals was the intra-subject repeatability that is due to the physiological stride to stride muscle activation variability.