A child car seat , comprising: a base configured to couple to a vehicle seat; a seat configured to couple to the base; and a sensor device mounted to one of the base or the seat, the sensor device including a sensor that outputs a measurement signal indicating whether a child is present in the seat, a processor configured to receive the measurement signal and to determine, based on the measurement signal, whether a child is present in the seat, and a communication circuit controlled by the processor, the communication circuit transmitting a proximity signal in response to the processor determining that a child is present in the seat, the proximity signal being configured for reception by a first mobile device ; wherein the proximity signal causes a first alert to be communicated to a user of a first mobile device in response to the first mobile device exceeding a range from the sensor device; and wherein a second alert is communicated to the user of a second mobile device if, after a first threshold amount of time, the measurement signal continues to indicate that a child is present in the seat and no response is received by the sensor device in response to the first alert; wherein the seat comprises a harness having a chest clip with a tongue and a buckle , the sensor device being housed within a housing of the tongue; wherein the processor is configured to receive an environmental measurement from an environmental sensor, the environmental sensor generating an environmental measurement representative of an environmental condition, and to determine whether the environmental measurement exceeds a maximum environmental measurement safety threshold; and wherein the processor is configured to respond to a determination that the environmental measurement exceeds the maximum environmental measurement safety threshold by reducing the first threshold amount of time. a base configured to couple to a vehicle seat; a seat configured to couple to the base; and a sensor device mounted to one of the base or the seat, the sensor device including a sensor that outputs a measurement signal indicating whether a child is present in the seat, a processor configured to receive the measurement signal and to determine, based on the measurement signal, whether a child is present in the seat, and a communication circuit controlled by the processor, the communication circuit transmitting a proximity signal in response to the processor determining that a child is present in the seat, the proximity signal being configured for reception by a first mobile device ; wherein the proximity signal causes a first alert to be communicated to a user of a first mobile device in response to the first mobile device exceeding a range from the sensor device; and wherein a second alert is communicated to the user of a second mobile device if, after a first threshold amount of time, the measurement signal continues to indicate that a child is present in the seat and no response is received by the sensor device in response to the first alert; wherein the seat comprises a harness having a chest clip with a tongue and a buckle , the sensor device being housed within a housing of the tongue; wherein the processor is configured to receive an environmental measurement from an environmental sensor, the environmental sensor generating an environmental measurement representative of an environmental condition, and to determine whether the environmental measurement exceeds a maximum environmental measurement safety threshold; and wherein the processor is configured to respond to a determination that the environmental measurement exceeds the maximum environmental measurement safety threshold by reducing the first threshold amount of time. <EOS>