How do I change the child inside a Photon Network instantiated gameobject so that the change is observed by all other users too?

This snippet doesn't work as expected:

  user =PhotonNetwork.Instantiate("User", new Vector3(0, 1, 0), Quaternion.identity, 0);
         if (PlayerPrefs.HasKey("username"))
         {
             user.transform.GetChild(1).GetComponent<TextMesh>().text = PlayerPrefs.GetString("username");
         }

What's wrong here and how do I correct it?