You will have to first add a Class to the Panel. The Class should be your PONumber. Here for my example I have used CustomerId.
<asp:GridView ID="gvCustomers" runat="server" AutoGenerateColumns="false" CssClass="Grid"
DataKeyNames="CustomerID" OnRowDataBound="OnRowDataBound">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<img alt="" style="cursor: pointer" src="images/plus.png" />
<asp:Panel ID="pnlOrders" CssClass='<%# Eval("CustomerID") %>' runat="server" Style="display: none">
<asp:GridView ID="gvOrders" runat="server" AutoGenerateColumns="false" CssClass="ChildGrid">
<Columns>
<asp:BoundField ItemStyle-Width="150px" DataField="OrderId" HeaderText="Order Id" />
<asp:BoundField ItemStyle-Width="150px" DataField="OrderDate" HeaderText="Date" />
</Columns>
</asp:GridView>
</asp:Panel>
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField ItemStyle-Width="150px" DataField="ContactName" HeaderText="Contact Name" />
<asp:BoundField ItemStyle-Width="150px" DataField="City" HeaderText="City" />
</Columns>
</asp:GridView>
Then you need to bind CustomerId to the DropDownList
<asp:DropDownList ID="DropDownList1" runat="server">
<asp:ListItem Text="Please select" Value="0"></asp:ListItem>
<asp:ListItem Text="ALFKI" Value="ALFKI"></asp:ListItem>
<asp:ListItem Text="ANATR" Value="ANATR"></asp:ListItem>
<asp:ListItem Text="ANTON" Value="ANTON"></asp:ListItem>
</asp:DropDownList>
Finally use the following script. It simply finds the Panel using CustomerId and displays it
<script type="text/javascript">
$("[id*=DropDownList1]").live("change", function () {
$("img[src*=minus]").closest("tr").next().remove();
$("img[src*=minus]").attr("src", "images/plus.png");
var className = $(this).val();
var img = $("img", $("." + className).closest("tr"));
$("." + className).closest("tr").after("<tr><td></td><td colspan = '999'>" + img.next().html() + "</td></tr>");
img.attr("src", "images/minus.png");
});
</script>
