C# - Add Children Controls Within A ServerControl Tags?

I'm working on a Navigation Menu. I've created below ServerControl and it works, but I want to allow users adding some standard ASP.NET controls within my ServerControl Tags like label, image and so on.

<MdsMenu:ServerControlMenu ID="ServerControlMenu1" runat="server">
<MdsMenu:animation AnimationSpeed="Normal" AnimationType="Opacity_Height" Delay="1000" DropShadow="true" />
<!-- HERE HAS TO HAVE SOME STANDARD ASP.NET CONTROLS -->
<!-- e.g <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label> -->
</MdsMenu:MenuItem>
</MdsMenu:ServerControlMenu>

My problem is here that how I can get Child Controls within <MdsMenu:MenuItem> and show them like as they are in the output. P.S: I overwrite RenderContents method

protected override void RenderContents(HtmlTextWriter output)
{
output.Write(OutPutStringBuilder.ToString());
}