Prefab clones behaviour

Hello everyone,

I don't get how prefab instantiation works. I'm coding a 2d vertical infinite scrolling and I made a script that continually create new tiles of background (prefab) at top and destroy itself when its at bottom. Then each background instantiate a group of windows (prefab too). Later some window would be animated or scripted or whatever so it is important that each window is a different gameObject.

The problem is, each background seems to herit windows from the last background created. So 1st bacnkground has 3 windows, 2nd has 6, 3rd has 9... etc..

like if all child were cloned too... but they dont exist in the prefab, I create them at runtime..

I'm sure I'm not using those prefabs the good way but that is my first Unity Project I don't how to handle the all-situation so if you have any clue..

here's my code :

 using UnityEngine;
 using System.Collections;
 
 public class bg_parallax : MonoBehaviour
 {
     static public Vector2         speed = new Vector2 (5, 5);
     static public Vector2         direction = new Vector2 (0, -1);
     public GameObject     background;
     public GameObject     window;
     private bool        needChild = true;
     // Use this for initialization
     void Start ()
     {
         ((GameObject)Instantiate (window, new Vector3 (-1.8f, -1.5f, 19f), transform.rotation)).transform.parent = transform;
         ((GameObject)Instantiate (window, new Vector3 (0f, -1.5f, 19f), transform.rotation)).transform.parent = transform;
         ((GameObject)Instantiate (window, new Vector3 (1.8f, -1.5f, 19f), transform.rotation)).transform.parent = transform;
     }
 
     // Update is called once per frame
     void Update ()
     {
     }
 
     void FixedUpdate ()
     {
         transform.Translate (new Vector3 (speed.x * direction.x, speed.y * direction.y, 0) * Time.deltaTime);
         if (needChild && transform.position.y <= 0) {
             ((GameObject)Instantiate (background, new Vector3 (0, transform.position.y + renderer.bounds.size.y, 20), transform.rotation)).transform.parent = transform.parent;
             needChild = false;
         }
         if (transform.position.y <= renderer.bounds.size.y * -1)
         {
             for(int i = 0; i < transform.childCount; i++)
                 DestroyImmediate(transform.GetChild(i).gameObject);
             Destroy (gameObject);
         }
     }
 }