Mouselook script beserk when GO made child

I've got a mouselook script that I'm relatively happy with, to aim a cannon. This all works fine if the cannon GameObject has no parent.

However, when I make the cannon the child of another gameObject, this happens:

As you can see, not only the Y rotation is affected, but the X and Z as well. The code is as follows:

     void Update () 
     {
         
         Plane playerPlane = new Plane(Vector3.up, transform.position);
         Ray hitRay = Camera.main.ScreenPointToRay(Input.mousePosition);
         
         float hitDist = 0.0f;
         
         if(playerPlane.Raycast(hitRay,out hitDist))
         {
             Vector3 targetPoint = hitRay.GetPoint(hitDist);
             
             Quaternion targetRotation = Quaternion.LookRotation(targetPoint - transform.position);
             
             
             
             Vector3 tempAngles = new Vector3(targetRotation.eulerAngles.x,ClampAngle(targetRotation.eulerAngles.y, maxAngle * -1, maxAngle), targetRotation.eulerAngles.z);
             
             targetRotation.eulerAngles = tempAngles;
             
             transform.rotation = Quaternion.Slerp(transform.rotation,targetRotation, 1); 
         }
     }
     
     
     
     public static float ClampAngle(float angle, float min, float max)
     {
                 
         if(angle > 180f)
         {
             angle -= 360f;
         }
 
         
         angle = angle % 180;
         
         return Mathf.Clamp(angle, min, max);
     }

The thought that just occurred to me is this: does a child object take values from its parent's transform? Because as I said, the X and Z axes are being affected, where they weren't before.

I'm completely flummoxed by this, any advice to get me started on troubleshooting would be awesome.