OBJECTIVE:​ During a frontal vehicle crash, for children seated in the rear seat, the first points of contact with the vehicle interior are often the foot, ankle, or tibia. The objective of this study was to evaluate leg interaction with the front seatback during a frontal collision using a prototype Hybrid III 6 year‐old anthropomorphic test device with more biofidelic lower extremity (ATD‐LE). METHODS:​ Eight frontal sled tests were conducted using a 48 km/h pulse. The front seatback properties were altered to simulate a rigid versus a soft front seatback condition. Four sled tests were conducted in each condition and within each condition, the ATD’s legs were either relaxed or extended and the front seat was either positioned forward or backward. RESULTS:​ The rigid front seatback condition recorded higher tibia forces, moments, and tibia indices. The seat in the back position with toes touching, resulted in the highest tibia bending moment and tibia index, 91.5 Nm and 1.4, respectively, both above injury threshold. Tibia moment injury threshold was also exceeded due to leg interaction with the front seat frame. DISCUSSION:​ These data suggest that front seatback material and rear compartment space are important determinants of anatomical and biomechanical interaction. These data provide evidence of potential sources of injurious loading, which may help determine areas in need of added protections in the future.