Alarm system, comprising a transmitter unit and a receiving unit operating in wireless mode, in which the receiving unit emits a luminous and/or acoustic and/or vibration alarm signal when the distance between said units (1, 2) exceeds a preset value, and in which the transmitting unit is provided with a switch whose state (A, B) determines the activation or deactivation of an alarm function of the alarm system, wherein - said transmitting unit is integral to the buckle of the harness of a children safety seat for motor vehicles, the harness comprising a predetermined number belts separately connected to the same buckle and to a plug that can be inserted and removably locked into the same buckle, the harness further comprising a locking group for keeping the plug locked into the buckle and a release button to release the plug from the buckle, - said switch is disposed in the state of activation of the alarm function by a force (Fl) exerted by the user on the plug for inserting it in the buckle and in the state of deactivation of the alarm function by a force exerted by the user on the release button to extract the plug from the buckle, such that the alarm function is activated automatically with the insertion of the plug into the buckle and is automatically deactivated with the extraction of the plug from the buckle by means of the release button, - the buckle comprises a plate, said plate having an anchoring lower side with a first slot for connecting it to the seat via a belt anchorage and, on the opposite side, a slit in which the plug can be inserted, the first slot and the slit being oriented orthogonally relative to one another, - the locking group comprises a body provided below the slit, said body being pushed upwards by a spring and assuming a raised position and respectively a lowered position when the plug is inserted into the slit or extracted therefrom, said body being located in correspondence of a second slot of the plate oriented parallel to the slit, - a transverse appendix of the body passes through the second slot, said appendix being connected with the switch, such that the displacement of said body determined by the insertion of the plug into the buckle, and respectively by its extraction, causes the activation or deactivation of the alarm function of the transmitter through the switch. - said transmitting unit is integral to the buckle of the harness of a children safety seat for motor vehicles, the harness comprising a predetermined number belts separately connected to the same buckle and to a plug that can be inserted and removably locked into the same buckle, the harness further comprising a locking group for keeping the plug locked into the buckle and a release button to release the plug from the buckle, - said switch is disposed in the state of activation of the alarm function by a force (Fl) exerted by the user on the plug for inserting it in the buckle and in the state of deactivation of the alarm function by a force exerted by the user on the release button to extract the plug from the buckle, such that the alarm function is activated automatically with the insertion of the plug into the buckle and is automatically deactivated with the extraction of the plug from the buckle by means of the release button, - the buckle comprises a plate, said plate having an anchoring lower side with a first slot for connecting it to the seat via a belt anchorage and, on the opposite side, a slit in which the plug can be inserted, the first slot and the slit being oriented orthogonally relative to one another, - the locking group comprises a body provided below the slit, said body being pushed upwards by a spring and assuming a raised position and respectively a lowered position when the plug is inserted into the slit or extracted therefrom, said body being located in correspondence of a second slot of the plate oriented parallel to the slit, - a transverse appendix of the body passes through the second slot, said appendix being connected with the switch, such that the displacement of said body determined by the insertion of the plug into the buckle, and respectively by its extraction, causes the activation or deactivation of the alarm function of the transmitter through the switch.