Disabling Children's GUI from script

You can get to child by pre-storing it from inspector variable, either get it using Transform.GetChild().

http://docs.unity3d.com/ScriptReference/Transform.GetChild.html

 public GameObject child; //store from inspector

 private void OnGUI(){
     if(GUI.Button(new Rect(0,0,100,30))){
         DisableChild();
     }
 }
 
 private void DisableChild(){
     transform.GetChild(0).gameObject.SetActive(false); //This will get the first child
     //OR
     //child.SetActive(false);
 }