A novel computer tool (peas@tees), designed to assess habitual physical activity levels in children aged 9 and 10 years, was evaluated. Study 1 investigated agreement between peas@tees and accelerometry in 157 children. Bland-Altman limits of agreement (LOA) revealed peas@tees underestimated physical activity levels compared with accelerometry (bias −21 min; 95% LOA -146–105). Study 2 investigated stability of peas@tees in a separate sample of 42 children. Intraclass correlation coefficient was 0.75 (95% CI 0.62–0.84). Computer tools are promising as a cheap, feasible, and useful method to monitor children’s habitual levels of physical activity at the group level.