add below code for your child
GridView
.
protected void GridView2_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "go")
{
GridViewRow Gv2Row = (GridViewRow)((LinkButton)e.CommandSource).NamingContainer;
GridView Childgrid = (GridView)(Gv2Row.Parent.Parent);
GridViewRow Gv1Row = (GridViewRow)(Childgrid.NamingContainer);
GridView Parentgrid = (GridView)(Gv1Row.Parent.Parent);
foreach (GridViewRow Grid1row in Parentgrid.Rows)
{
Label lbl = (Label)(Grid1row.FindControl("Label1"));
string grid2Rowtext = lbl.Text;
Response.Write(grid2Rowtext + "<br>");
}
}
}