Fill child gridview(nested) from parent gridview listbox selected index changed event

i want to fill the child gridview(nested) with empty textboxes when listbox under parent gridview is fired (items selected).i wanted an example of that thing to implement in my project. i want to bind the empty textboxes based on number listbox items selected in parent gridview.

What I have tried:

i have tried by finding child gridview but that could not do the trick.