Insufficient physical activity (PA) among most children and adolescents is a global problem that is undermining the realisation of numerous developmental and health benefits. The aim of this study was to explore the potential impact of interventions on PA by using an agent-based model (ABM) simulating children’s mobility and daily activities in an urban environment.  ABMs are computational models that simulate complex social systems by representing agents that interact with one another and with the environment in which they live according to predefined rules. The micro-level behaviours of agents generate the dynamics of the system from the bottom-up and lead to the emergence of macro-level patterns observed at the population level. 
In this study, we formalise our understanding of the multiple interacting factors that form children’s PA system by means of an agent-based model (ABM). We use the ABM to simulate the emergence of PA levels of children residing in an urban environment and evaluate the potential impact of several interventions in children’s daily activity on PA levels. 
The ABM simulates typical daily activities of children, such as travel, school lessons, sport, outdoor play, meeting friends and shopping, during which physical activity is accumulated. The agents are embedded in an urban environment represented by geographical information data. Using the model, we explored how factors related to agents’ characteristics, their interactions and the environment to which they were exposed impact PA. The model explores three domains for interventions: outdoor play, school physical education and active travel. 
Simulation results show that active travel constitutes a quarter of the daily moderate-to-vigorous-intensity PA. Promotion of active travel and outdoor play benefited more those in a higher socio-economic position. Agents’ interactions suggested that: encouraging activity in diverse groups will reduce the percentage of the least active in the population; and initiating outdoor events in neighbourhoods can generate an enhancing effect on children’s engagement in PA. The ABM provided measurable outcomes for interventions that are difficult to estimate using reductionist methods. We suggest that ABMs should be used more commonly to explore the complexity of the social-environmental PA and mobility systems.