using convertToGL() to sub-node?

using convertToGL() to sub-node?
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I found that the convertToGL() helps me to convert the origin (0,0) to TopLeft of the screen which is somehow useful.
However, when I have a child under a translated parent (e.g. button inside a dialog frame or an HP bar over a monster’s head ), I can’t use convertToGL() to the child.
The convertToGL() seems just return (screen height - y) as y-coordinate to me. Thats pretty useless in this case.

What is the right way to do it?
Or should I just not use convertToGL() at all?