One child I was married to their mum so didn’t need to legitimize, the other we went to court to legitimize, no lawyers used , they would of only been used as translator if I had used one , it was easy , but had to go back to court to get another paper from them , cost all around 800 baht ,
the first paper the amphur would recognize , 50 baht for the second paper , sorry I don’t know what the papers are called but the family courts are so helpful , worth asking them