When children have to travel abroad, some documents may be required at the airport, depending if you are the legal parent, the legal guardian or another party. And it could be different from a country to another. Always verify before if you are not sure.
Below is an example of letter that was done in Thai language and signed at the amphur. It was translated in English and we deleted the names. This letter was for the American father of a Thai child that was unmarried with the mother.