I am using Nested GridViews where each row in the gridview has child gridView. I am using RowDataBound Event of Parent GridView, to Bindin
I don't think you will be able to track it normally, but I would embed ID field into the hidden field and put this hidden field under TemplateField,
this way you can get its value by going
gvChild.Parent.FindControl("idOfYourHiddenField");