C# - How To Render Children Tags In A Custom Server Control

I'm working on a custom ServerControl, I've created it like below :

[ParseChildren(true), PersistChildren(true)]
[ToolboxData("<{0}:Menu runat="server"></{0}:Menu>")]
public class Menu : WebControl
{
.....
[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
[PersistenceMode(PersistenceMode.InnerDefaultProperty)]
public MenuItem MenuItems { get; set; }
}
[ParseChildren(true), PersistChildren(true)]
public class MenuItem : WebControl
{
......
[PersistenceMode(PersistenceMode.InnerProperty)]
public MenuItem SubMenuItems
{
get
{
if (_SubMenuItems == null) return new MenuItem();
return _SubMenuItems;
}
set
{
_SubMenuItems = value;
}
}
private MenuItem _SubMenuItems;
[TemplateContainer(typeof(MenuItem))]
[PersistenceMode(PersistenceMode.InnerProperty)]
public ITemplate Template { get; set; }
}
<%@ Register Assembly="JQueryMenu" Namespace="JQueryMenu" TagPrefix="MdsMenu" %>
<MdsMenu:Menu ID="Menu1" runat="server">
<AnimationItems AnimationSpeed="Fast" AnimationType="Opacity_Height" DropShadow="true"
Delay="1000" />
<MdsMenu:MenuItem ID="MenuItem1" runat="server" Text="MenuItem 01">
<MdsMenu:MenuItem runat="server">
<Template>
<asp:Button ID="Button1" runat="server" Text="Button" />
<asp:CheckBox ID="CheckBox1" runat="server" />
</Template>
</MdsMenu:MenuItem>
</MdsMenu:MenuItem>
<MdsMenu:MenuItem ID="MenuItem2" runat="server" Text="MenuItem 01">
<MdsMenu:MenuItem ID="MenuItem3" runat="server">
<Template>
<asp:Button ID="Button2" runat="server" Text="Button" />
<asp:CheckBox ID="CheckBox2" runat="server" />
</Template>
<MdsMenu:MenuItem ID="MenuItem5" runat="server" Text="MenuItem 05">
</MdsMenu:MenuItem>
<MdsMenu:MenuItem ID="MenuItem6" runat="server" Text="MenuItem 06">
</MdsMenu:MenuItem>
<MdsMenu:MenuItem ID="MenuItem4" runat="server">
<Template>
<asp:Image ID="Image1" runat="server" />
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</Template>
</MdsMenu:MenuItem>
</MdsMenu:MenuItem>
</MdsMenu:MenuItem>
</MdsMenu:Menu>

Now, How can I Parse it and render it in RenderContent method ? !!!
The following method is always throw the first Exception, it means this.Controls is always empty !!! How can I do it and how I can access to the nested children in RenderControl method ?

public class Menu : WebControl
{
....
public override void RenderControl(HtmlTextWriter output)
{
if (!this.HasControls())
throw new Exception("Controls are empty");
....
}
}