How to rotate a child object independently from it's parent so that it maintains it's transform integrity and shape

I have a rotatable parent object, that rotates using transform.Rotate(), and also a to be rotatable child object.

what I want to achieve is for the child object to rotate with the rotation of it's parent, which is fine, but also rotate on it's own, which results in some weird deformity to the shape of the child. as in the picture

code parent

 public class Platform : MonoBehaviour {
 
 
 
     public float rotation;
     public float force;
 
     private Quaternion origin;
     private Rigidbody rigidBody;
     // Use this for initialization
     void Start () {
         origin = transform.rotation;
         rigidBody = GetComponent<Rigidbody> ();
     }
     
     // Update is called once per frame
     void Update () {
     
         float forward = Input.GetAxis ("Vertical");
         float side  = Input.GetAxis ("Horizontal");
     
         transform.Rotate (new Vector3 (forward * rotation,0,-side * rotation));
 
 
         transform.rotation = Quaternion.Slerp (transform.rotation, origin, .1f);
 
 
         if (Input.GetKey (KeyCode.O)) {
             print ("force");
             rigidBody.AddForce (transform.up*force, ForceMode.Force);
 
         }
     }
 }
 

code child

 public class player : MonoBehaviour {
 
     public Transform sun;
     public float rotation;
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
 
         float forward = Input.GetAxis ("Vertical");
         float side  = Input.GetAxis ("Horizontal");
 
         transform.Rotate (new Vector3 (forward * rotation,0,-side * rotation));
     }
 }