Parents are being urged to equip their vehicles with seats and seatbelts suitable for young children to prevent a repeat of Sunday's accident in which a six-year-old boy was thrown from a pickup truck on an elevated expressway and fell to his death in Samut Prakan.

The accident sent the boy flying out of the open window to his death 30 metres below and prompted National Institute for Child and Family Development of Mahidol University director Dr Adisak Plitponkarnpim to lead calls for parents to obtain car seats for their young.

Dr Adisak said that despite the law that children under six years old or whose height is below 135 centimetres must wear a child seatbelt or use a safety seat, most parents overlook the matter.

When any vehicle travelling at 80-100 kilometres per hour suddenly has to brake, objects and people sitting inside will lunge forward at the same speed, he said.

However, regular seat belts are not appropriate for young children, and no cars could be considered or advertised as "family" vehicles unless they are installed with car seats for child passengers, according to Dr Adisak.

In pickup trucks, child safety seats should be strapped into the front passenger seat with the airbag disabled first to prevent them causing impact injuries.

"The government should reduce import tax on safety seats for children to make them affordable for all families," said Dr Adisak.