This study presents an observational system to quantify the duration, intensity, and frequency of children's physical activities under natural conditions. The system was tested on 15 children aged 6–10 years in southern California, with observations recorded every 3 seconds during 4-hour time blocks from 8:00 a.m. to 8:00 p.m. Observers achieved 91% agreement in coding. Using indirect calorimetry, the system was calibrated to determine energy expenditure for each activity, categorizing them as low, medium, or high intensity. Children spent 77.1% of their time in low intensity activities and 3.1% in high intensity activities. The median duration of low and medium intensity activities was 6 seconds, while high intensity activities lasted only 3 seconds, with 95% lasting less than 15 seconds. Children engaged in short bursts of intense activity interspersed with varying intervals of low and moderate intensity. These findings suggest that children's activity patterns under natural conditions significantly influence physiological processes related to growth and development. The study highlights the importance of capturing not only the intensity and frequency of activities but also their duration and intervals between activities. The observational system provides a comprehensive assessment of children's physical activity, including the tempo of energy expenditure, which is defined as the pattern of activity in terms of frequency, duration, and intensity. The study also discusses the limitations of the system, including the need for extensive training and data collection, and suggests its potential use as a validation tool for other field assessments. The results indicate that children spend most of their time in low intensity activities, with high intensity activities being brief and infrequent. The study underscores the importance of understanding the rapid changes in children's physical activity patterns and their physiological implications for growth and development.This study presents an observational system to quantify the duration, intensity, and frequency of children's physical activities under natural conditions. The system was tested on 15 children aged 6–10 years in southern California, with observations recorded every 3 seconds during 4-hour time blocks from 8:00 a.m. to 8:00 p.m. Observers achieved 91% agreement in coding. Using indirect calorimetry, the system was calibrated to determine energy expenditure for each activity, categorizing them as low, medium, or high intensity. Children spent 77.1% of their time in low intensity activities and 3.1% in high intensity activities. The median duration of low and medium intensity activities was 6 seconds, while high intensity activities lasted only 3 seconds, with 95% lasting less than 15 seconds. Children engaged in short bursts of intense activity interspersed with varying intervals of low and moderate intensity. These findings suggest that children's activity patterns under natural conditions significantly influence physiological processes related to growth and development. The study highlights the importance of capturing not only the intensity and frequency of activities but also their duration and intervals between activities. The observational system provides a comprehensive assessment of children's physical activity, including the tempo of energy expenditure, which is defined as the pattern of activity in terms of frequency, duration, and intensity. The study also discusses the limitations of the system, including the need for extensive training and data collection, and suggests its potential use as a validation tool for other field assessments. The results indicate that children spend most of their time in low intensity activities, with high intensity activities being brief and infrequent. The study underscores the importance of understanding the rapid changes in children's physical activity patterns and their physiological implications for growth and development.