How to make the child follow the parent's rotation

I have a top-down game with a camera looking flat to the ground (which means it's tilted 90 degrees downwards). I am making a turret rotation that follows the location of the mouse.

I have successfully made it look at the mouse in a flat manner but whenever I go to ramps, it doesn't rotate as the parent as you can see on the screenshots.

Current ship orientation:

Zoomed in to turret:

I have a script on an Empty Game Object which is the parent of the cannon. I currently have this line of codes in the Update() function

 Vector3 v3T = Input.mousePosition;
 v3T.z = Mathf.Abs (Camera.main.transform.position.y - transform.position.y);
 v3T = Camera.main.ScreenToWorldPoint (v3T);
 transform.LookAt (v3T);

How do I make the Empty Game Object to follow the parent's (I'm referring to the ship) rotation?