The safest way for children to travel in cars is in a child seat that is suitable for their weight and size.
Even in a minor crash, an unrestrained child would be thrown about inside the vehicle, injuring themselves and others. They could be thrown from the car through one of the windows. Click on the link below for advice on keeping your children safe when they travel in cars.
What the law says*
The law says that all children up to 135cm tall (around 4'5"), or the age of 12, whichever comes first, in the front or rear seats in cars, vans and other goods vehicles must travel in the correct child restraint for their weight with very few exceptions.
Visitors to the UK from abroad must also use the correct child restraint for their children - there are no exceptions for them. Family and friends expecting visitors should make sure that their visitors understand the rules. They may need to help them make arrangements so that children use the correct child restraint at all times. Car rental companies can supply child restraints for their customers if booked in advance.
Exceptions to the rules
In limited circumstances, children can travel without the correct child restraint.
1) In a licensed taxi or licensed private hire vehicle. If the correct child restraint is not available then, in the rear seat only, children under three may travel unrestrained. Children aged three years and over must use an adult seat belt.
2) In cases of unexpected necessity over a short distance. If the correct child restraint is not available then, in the rear seat only, a child of three years or more must use an adult belt. This exemption does not apply to children under three years and does not cover regular school runs or other journeys that are planned in advance.
3) Where two occupied child restraints in the rear seat prevent you fitting a third. In this case, provided the front seat is occupied, a third child aged three years and over can use an adult seat belt (lap OR lap and diagonal) in the rear. If the front seat is free, then they must sit there using the correct child restraint.
4) In older vehicles with no rear seat belts. In this case, children three years and over may travel unrestrained.
* source: http://www.dft.gov.uk/