A modular child restraint system for use in vehicles comprises a base (100) and interchangeable attachable seats (110, 120) where the seats are for different stages of childhood development. The system can further comprise additional bases adapted to other uses such as strollers and infant swings. A common engagement mechanism allows the various seats to be moved from one base to another. One such system comprises a base (1010) and a seat configured to rotate relative to one another when engaged. The seat can also lock in a position relative to the base. The base includes a visual indicator (1110) to show that the seat is secured to the base.