Custom pivot for a gameobject

Below is a picture depicting "I", "J", "L" game objects. Each object has children. These children are blocks and a pivot.
I want to instantiate game objects relative to their child Pivot, but I do not how to do it.

I have a script that instantiates these objects

Code (CSharp):

1.  
2.     public GameObject[] Shapes;
3.     public Vector2 beginPoint;
4.     public Vector2 resize;
5.     public float offsetBetweenBlock;
6.     private const int numberToShow = 3;
7. ...
8. for(int i = 0; i < numberToShow; i++)
9.         {
10.             GameObject currentShape = Shapes[Random.Range(0, Shapes.Length)];
11.             Instantiate(
12.                 currentShape,
13.                 new Vector3(beginPoint.x + i * offsetBetweenBlock, beginPoint.y),
14.                 Quaternion.identity,
15.                 transform
16.                 );
17.         }

But as you can see "J"(Medium) and "L"(Right) are not equal to each other (I pointed at them with red arrows on the picture)

How can I instantiate objects "I", "J", "L" with respect to their child Pivot?

 

Thanks! It is so easy. Sorry for my question