C# - How To Render User Control Children At A Specific Location

I have a very simplistic user control that looks like this:

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="wfWindow.ascx.cs" Inherits="webfanatix.co.za.wfWindow" %>
<div>Just a test...[x]</div>

with this code behind:

[ParseChildren(false)]
[PersistChildren(true)]
public partial class wfWindow : System.Web.UI.UserControl
{
protected override void Render(HtmlTextWriter writer)
{
RenderChildren(writer);
}
}

And the usage thereof looks like this:

<wf:wfWindow runat="server">This content should go where [x] is.</wf:wfWindow>

I'm no ASP.NET pro, so how do I get the content to render exactly where the [x] appears in my user control?

RenderChildren is rendering my content, but it is only appended to the end of the UserControl output. I need it to go and sit right where [x] marks the spot.