When driving abroad the driver and passengers must wear a seatbelt, where one has been fitted. The driver of the car is responsible for any passengers under the age of 18. These passengers should have the correct safety restraint, car seat if required, when travelling. For passengers over the age of 18 it is their responsibility to ensure they are wearing a seatbelt.
When you are in your hire car and on holiday it can be easy to overlook safety issues, however please be aware that it is the law to wear a seat belt in your rental vehicle, just as it is back home. Some countries may even fine passengers for not wearing a seat belt. For example, in Germany a fine of €30 may be imposed and up to €40 if a child is found to be unsecured.