While emphasising play-based approaches in early childhood education, there is a deficiency in the practical understanding of play integration into children’s learning environment design. To support methodological limitations, this study developed a play behaviour model using reinforcement learning agents to generate children’s unpredictable behavioural responses during physical and social play, including unforeseen injuries. In comparative laboratory experiments, 14 professional architects engaged in design tasks before and after using the simulation. The design results were assessed through external expert evaluations, with a focus on the optimal trade-offs between facilitating play behaviours while ensuring safety considerations and on the level of effective design decision making and completeness. The statistical analysis demonstrated that the simulation facilitated optimal trade-offs, enhanced decision making and improved the overall completeness of children’s learning environment design. This research addressed the capabilities of simulation to support a child-centric design, with potential implications for improving early childhood education environments.