This case study describes a fatal head injury in a 1-year-old child involved in a motor vehicle accident in Japan. The child, secured in a rear-facing child seat, was a passenger in a car driven by their mother when the offset car-to-car collision occurred. The car rotated counterclockwise before coming to a stop. Despite remaining secured in the child seat, the child suffered severe head trauma, leading to cardiac arrest. Autopsy computed tomography revealed a right open depressed fracture, left head contusion, traumatic subarachnoid hemorrhage, intraventricular hemorrhage, and pneumocephalus. The injury mechanism involved the child’s head striking the right headrest, followed by a swing to the left, induced by the initial impact and subsequent rotational movement. This case highlights the importance of age-specific data in understanding pediatric injuries in motor vehicle accidents and improving child seat safety measures.