Traveling safely in motor vehicles can be challenging for many families who have young children with physical disabilities. Harnesses, simple adaptations, and special child restraint systems are available, but sometimes these devices do not adequately meet the unique postural support requirements of children with complex seating needs. Faced with no alternative, parents may choose to use the custom seating system from a wheeled mobility device to support their children in the family car. Transporting children in this way can increase the risk of motor vehicle-related injury because custom seating systems are not designed to meet the requirements of federal motor vehicle safety regulations. We studied whether assistive technology suppliers could build custom child restraint systems that met the crashworthiness requirements of a safety standard for production child restraint systems. We provided technical instructions to 10 suppliers from different parts of North America so they could each build a custom restraint system using a transit frame that we designed. This approach allowed suppliers to make custom seats that could be attached to the transit frame using special connection hardware. We crash tested the 10 custom child restraint systems to evaluate the effectiveness of our transit frame design and fabrication instructions. Six custom restraint systems met the dynamic performance requirements of the stringent Canada Motor Vehicle Safety Standard 213.3. The remaining four systems did not meet the compliance criteria due to the failure of postural belt assemblies or seat securement hardware. We recommend that future research include similar effectiveness studies to support the introduction of technical requirements for adaptive seating systems that improve occupant safety and are practical for wheelchair users, their families, and assistive technology professionals to implement.