Objectives Research has shown that having adequate quantity and quality of physical activity can contribute to the health and well-being of children. Nonetheless, existing tools to measure these constructs in children have limitations in terms of their objectivity and scalability. In this study, we provide criterion validity evidence of two systems built on commercially available sensors (ie, gyroscopes and infrared cameras), designed to measure children's moderate-to-vigorous physical activity and fundamental movement skill proficiencies. Design Cross-sectional. Setting Primary schools in Hong Kong. Participants Data from 30 (age=8.55±1.25 years) and 1174 (age=9.15±1.63 years) children were included for the validation of physical activity and fundamental movement skills measures, respectively. Children's outcomes were simultaneously measured using the developed systems and existing, well-established measures (accelerometers and expert ratings). Results We found a strong correlation between physical activity outcomes measured using our developed system and accelerometers (Pearson r=0.795). Motor skill proficiency scored using our real-time rating system had strong agreement with expert ratings (percentage agreement=84%-94%, kappa=0.661 to 0.859). Discussion Results of the current study supported the application of the respective systems in physical education and large-scale research studies. Collection of such data at mass levels could help researchers depict the complex relation between children's quantity and quality of physical activity. Copyright © 2022 Author(s) (or their employer(s)).