You can't override any function with scripts, if it's not marked virtual in the docs. You can't override properties either, at least I think it's not intented to work.
If you want a child node to keep its rotation, you have a few options:
1) Not make it a child of the rotating part, though you might want to keep it in the same scene. You could eventually unparent the child at runtime using a script.
2) Rotate a child and not the parent, especially if the rotation is only for visual purpose. Typically, you could do that if you separate the visual part of your scene in a distinct branch so that you get more control over its transforms. I usually go for that solution.
3) Add an intermediate Node between the parent and the child (so you have parent -> node as child -> your non-rotating node as sub-child). However, doing so makes the sub-child behave as if it was in world space so you will need to move it manually.
4) Don't use nodes, use _draw or servers directly and calculate transforms yourself. This is a bit too advanced for that use case though.