The XL vest is certified to 110lbs. That certification is based on the lap-only belt and tether configuration.
If someone is using the RideSafer and the vehicle’s lap-shoulder belt the RideSafer will continue to do what it is designed to do and properly pre-crash position the seat belt. Even though they would technically be outside manufacturer specs once they understand the design limits they have a parental choice to make.
Sometimes another product is the best solution. If a restraint bigger than the XL extra is required, we would recommend a Merritt Manufacturing Churchill restraint.