これだとエラー
exp.cs
GameObject prefab = (GameObject)Resources.Load("Prefabs/UI/Balloons/BtnBalloon") as GameObject;
Destroy(prefab,4f);
NGUITools.AddChild(BalloonParentObj, prefab);
これだとOK
exp.cs
GameObject prefab = (GameObject)Resources.Load("Prefabs/UI/Balloons/BtnBalloon") as GameObject;
GameObject BalloonPrefab = NGUITools.AddChild(BalloonParentObj, prefab);// NGUIに追加 帰り値で、prefabの実体を参照
Destroy(BalloonPrefab,4f); //4秒後に追加したprefabを削除