Booster Seat Safety Act This bill addresses labeling requirements and testing standards for booster seats used in motor vehicles. Specifically, the bill requires labels on booster seats to include the minimum recommended age (4 years) and weight (40 pounds) for a child to use the seat. It also requires that a label on a car seat or combination car seat list the maximum weight and height limits of the seat, as well as a recommendation to continue using the seat until the child exceeds the listed limits. Additionally, the National Highway Traffic Safety Administration (NHTSA) must either establish a minimum height recommendation for booster seats or a method for manufacturers to use to determine a minimum height recommendation. Once the minimum height or method is established, booster seat labels must also contain the minimum height recommendation. The NHTSA must also establish a standard for side-impact crash tests for booster seats and provide guidelines for creating a testing dummy that approximates a six-year old child for such crash tests. Further, the NHTSA must study systems for tethering child restraint systems in vehicles and make recommendations about using or modifying tether systems to maximize child safety.