unity instantiate prefab with child code example

Example 1: unity instantiate prefab

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

Example 2: 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