All children must travel in the car in an appropriate car seat until they are either 12 years old or 135cm tall
For adults and children aged 14 and over it is their own responsibility to make sure they wear the seat belt. But it is the driver's legal responsibility to ensure that all children under the age of 14 are correctly restrained in their car, even if the child's parents are passengers in the same car.
Babies and toddlers under the age of three must travel in an appropriate car seat in any private vehicle, whether it's your own or someone else's. You must never travel in a car with a baby or toddler on your lap under any circumstances! The ONLY exception for under-threes is that they can legally travel in a taxi on an adult's lap.
For children aged between three and 12 there are four exceptions to the law, which mean that in any of these four circumstances a child over three is legally permitted to travel in the car without a car seat. These exceptions only exist for practical reasons, a child is never safe without a car seat, so you should try to avoid these situations whenever possible and make it a priority to use a car seat.
The four exceptions are: 1. Travel in a taxi or minicab 2. When there are two occupied child seats in the car and a third won't fit 3. When there are no seat belts fitted in the back of the car 4. Short journeys of unexpected necessity in someone else's car