it can't be done if the child was born post 1992. Both foreign parents needed to have PR at the time of the childs birth for the child to be a Thai citizen by birth.
So far as I understand, children born before 1971 in just about all cases are Thai citizens by virtue of being born on Thai soil, regardless of the parents nationality.
Children born in Thailand between 1971 and 1992 to non-Thai parents have the option of accessing Thai nationality so long as that child can show a long term association with the country.