The requirement is being made compulsory under the Land Traffic Act, reports the Bangkok Post.
Section 123 of the law stipulates that children under six years old must be restrained in a car seat or a special seat for safety in a moving vehicle.
Details will be fleshed out in a specific regulation to be issued by the national police chief.
The law provides an exception for children who cannot be fastened to a seat for health or physical reasons.
Nikorn Chamnog, deputy chairman of the House committee which scrutinised changes to the Land Traffic Act, said since car seats are costly, retailing mostly at more than B10,000 apiece.
Because of that, the amended law allows the use of less expensive seat cushions priced around B600-700 each.
The cushion enables the seat occupant to sit high enough for fastening with effective restraint, said Mr Nikorn, who is also chairman of the World Health Organization’s Asia-Pacific Regional Network on Road Safety.
The House committee studied the mandatory introduction of car seats in the Philippines and Malaysia, finding the requirement proved chaotic and difficult in early phases due mainly to the high price of car seats.
Mr Nikorn said motorists will need time to adjust, so police should be lenient regarding compliance and issue warnings at first rather than taking more serious action during the early phase of enforcement after Sept 5.
At the same time, however, a campaign should be launched to educate people about the measure and the steps needed.
Mr Nikorn said the government likewise should also consider lowering the import tax for car seats while promoting local production of the seats.