Hello,
I did the non o at the Thai embassy in london based on children under 20 (as I’m now divorced)
you need 10,000 gbp - I only shown them a months statement.
I have never done the visa in country but it is of course possible, a friend of mine did it and from what I can recall the child had to go down with him.
If you are not married to your child’s mother this maybe an issue as under thai law you do not have any parental power unless you marry her or court grants it.