Abstract: A child safety seat system and the corresponding method of using a modular child safety seat assembly to compensate for the stature changes of a growing child. The child safety seat system utilizes a base shell that is strapped into an automobile using the automobile seat belt restraints. A plurality of interchangeable seating inserts are provided. Each of the seating inserts has a cushioned seat. The thickness of the cushioning used depends upon the targeted age of a child and is engineered to maintain the child's shoulders within a specific range of heights relative to the base shell. An attachment mechanism is used to attach a selected seating insert to the base shell. As a child outgrows a seating insert, that seating insert is removed from the base shell and is replaced with a seating insert sized for a larger child.