How to remove all children of panel in update panel

Hello everyone.

I has a problem.. In pageload event i create dynamic textbox and add to panel (pn_control).. When i click button i want to remove textbox and add new dropdowlist into pn_control. I use update panel, it don't work but when i remove update panel in code design, it work..

So.. how can i remove all children of panel using updatepanel?

Here is my code.

<table style="width: 100%;">
                    <tr>
                        <td align="right" style="padding-right: 10px;">
                            <asp:UpdatePanel ID="UpdatePanel3" runat="server" UpdateMode="Conditional">
                                <ContentTemplate>
                                    <asp:Button ID="cmd_ViewReport" runat="server" Width="90" Text="Xem báo cáo" OnClick="cmd_ViewReport_Click" />
                                </ContentTemplate>
                            </asp:UpdatePanel>
                        </td>
               
                    <tr>
                        <td>
                            <asp:UpdatePanel ID="udp_Control" runat="server" UpdateMode="Conditional">
                                <ContentTemplate>
                                    <asp:Panel ID="pn_Control" runat="server" Style="width: 100%; padding: 5px 5px;">
                                    </asp:Panel>
                                </ContentTemplate>
                                <Triggers>
                                    <asp:AsyncPostBackTrigger ControlID="cmd_ViewReport" EventName="Click" />
                                </Triggers>
                            </asp:UpdatePanel>
                        </td>
                    </tr>
                </table>

CS:

protected void cmd_ViewReport_Click(object sender, EventArgs e)
    {
        pn_Control.Controls.Clear();
        DropDownList ddl = new DropDownList() { ID = "ddl_Demo", Width = 150 };
        pn_Control.Controls.Add(ddl);
    }