A weight-sensitive mat or other sensor determines whether a child seat in a vehicle is occupied, a transmitter is proximate to the sensor, and a keychain fob or other portable unit includes a receiver and an alarm. Generally, if the child seat is occupied and the keychain fob is removed from proximity to the transmitter, the transmitter communicates this to the keychain fob receiver and the alarm is activated. Exemplary embodiments include an add-on kit including a base unit with a transceiver for use with existing car seats having a base and a detachable shell, an add-on kit without the base unit for use with existing unitary car seats, a car seat with the child sensor, transmitter, and/or base unit built into it, and an automatic checklist feature for diaper bags, strollers, etc.