In indoor evacuations, children may use three-dimensional (3D) motions (e.g., low crawling, climbing up/down) to navigate above or below indoor objects within constrained spaces or under higher urgency levels. Understanding how these motions influence children’s evacuation efficiency benefits the development of behavioural instructions. This study aims to investigate the influence of 3D motions on the efficiency of children’s indoor evacuations. We employ a simulation method that integrates a voxel-based 3D indoor model with an agent-based model. Detailed scenarios are elaborated to account for agent numbers, urgency levels, and physical attributes. These scenarios serve as exploratory demonstrations rather than validated simulations grounded in real-world evacuation experiments. Findings are: 1) The use of 3D motions is relatively low across all urgency levels and agent numbers. 2) The number of agents moving above or below indoor objects slightly increases with higher urgency and more agents. 3) 3D motions may not significantly influence children’s evacuation efficiency due to minor congestion, limited use of 3D motions, and lower speeds of 3D motions. We suggest further investigation into behavioural instructions, including 1) Limit the use of 3D motions in low-density scenarios. 2) Conditional use of 3D motions when local congestion significantly hinders direct evacuation paths. 3) Conduct evacuation exercises tailored to children’s physical and cognitive abilities under specific scenarios. This study fosters immediate evacuation response strategies, long-term preparedness, and educational measures for children.