How to declare/tie a mdigrandchild form to mdiGrandParent form?

How to declare/tie a mdigrandchild form to mdiGrandParent form?

Situ:
I'm writing some code/mdiParent that spawns a child form, child1, that has all the information capturing. Now child1 spawns a child, grandchild. The grandchild is a lean version of child1, readonly type situation. Child1 is not shown, but working. Grandchild is to: Grandchild.show().

Needed:
I need to declare the grandchild to show in the mdiParent. I'm sure it's somthing similar to :

The parent is MainIndiv_Truck_ControlMDI : Form

the child1 is declared in the parent as:

Indiv_Truck_Control TruckForm = new Indiv_Truck_Control();
TruckForm.MdiParent = this.

I know I'm missing something. I can make the grandchildren, but there happening outside the MdiParent. I want them to happen inside MainIndiv_Truck_ControlMDI : Form

Thanks in advance.
L.