Answers for "unity instantiate prefab as child"

unity instantiate prefab

Instantiate(cube, Vector3 (x, y, 0), Quaternion.identity);

unity instantiate prefab as child

//Instantiate Object
     GameObject go = Instantiate(A, new Vector3 (0,0,0), Quaternion.identity) as GameObject;
     // Tranform as child
     go.transform.parent = GameObject.Find("Stage Scroll").transform;

unity create prefab with child

Add the original prefab into the scene, add the new prefab as a child to it, 
then drag the original prefab back onto the prefab in the project view (or just
click Apply in the inspector)
Probably want to delete it from the scene after that