@Alireza ***
. To put the agents price Into perspective.
*****
baht is more than the agent would earn in a month. You provide the documents to them anyway , all they do is fill out application form and deliver paperwork to immigration. Which they do in bulk, not one at a time. If you are prepared to pay that much for someone to do maybe 2 hours work tops, then the other option is to take a day off work and do it yourself for 1900 baht per child.And are you happy handing over your passport and your children's passports to total strangers for as long as it takes? They normally make you wait until they have a pile of them before going to imm themselves. Immigration always return your passport immediately if you apply in person.