ClientId for a control in a user control

I am working on a tree view control for a project that I am currently involved in. This tree view is made of three parts: TreeNode class, TreeNodeCollection class, and TreeView user control. The control has one line of HTML, a div container. When I call a function, I need it to run through the tree node collection recursively and generate the links and buttons necessary. Ok, well I've got the code generating all of the links and buttons, but the actually hiding of the nodes isn't happening because the javascript requires the id of the containing div. I am coding it to take the ClientId of the div as I create it, but it is only giving me the id that fed to the control, and not the one used when the page is generated. This is doing some screwy stuff, so if you want to give me some pointers elsewhere, I'ld definitely appreciate it.

I'm not going to post all of the code, just what I find to be the problem areas.

TreeView.ascx html
<%@ Control Language="vb" AutoEventWireup="false" Codebehind="TreeView.ascx.vb" Inherits="PeCON.TreeView" TargetSchema="http://schemas.microsoft.com/intellisense/ie5" %>
<div id="TreeViewContainer" runat="server" style="text-align: left"></div>


TreeView.ascx.vb
    Public Sub BuildTreeView()
        ' call recursive method for displaying nodes

        'TreeViewContainer is defined in the Windows Generated Code section but is not instantiated when we come across this code
        If IsNothing(TreeViewContainer) Then
            TreeViewContainer = New Web.UI.HtmlControls.HtmlGenericControl("div")
            Me.Controls.Add(TreeViewContainer)
        End If

        DisplayNodes(_nodes, TreeViewContainer, 0)
    End Sub

    Private Sub DisplayNodes(ByVal Nodes As TreeNodeCollection, ByVal Container As Web.UI.Control, ByVal Level As Integer)
        For i As Integer = 0 To Nodes.Count - 1
            Dim hl As New Web.UI.WebControls.HyperLink
            hl.Text = Nodes(i).Name
            hl.NavigateUrl = Nodes(i).Url
            Container.Controls.Add(hl)

            Dim newLine As New Web.UI.WebControls.Literal
            newLine.Text = "<BR>"
            Container.Controls.Add(newLine)

            If Nodes(i).HasChildren Then
                Dim btn As New Web.UI.WebControls.Image
                btn.ID = "btn" & i.ToString & "_" & Level.ToString
                btn.ImageUrl = "..\images\plus.gif"
                Container.Controls.Add(btn)

                Dim newDiv As New Web.UI.HtmlControls.HtmlGenericControl("div")
                newDiv.ID = "div" & i.ToString & "_" & Level.ToString
                newDiv.Style.Add("text-align", "left")
                newDiv.Style.Add("margin-left", CStr(Level * 0.2) & "in")
                DisplayNodes(Nodes(i).Children, newDiv, Level + 1)
                Container.Controls.Add(newDiv)

                btn.Attributes.Add("onclick", "ToggleDisplay(this, " & newDiv.ClientID & ")")
            End If
        Next
    End Sub


Generated output:
<div id="mainContent_divContent" align="center" class="ContentPane">
      <img class="ContainerTopLeft" src="images\cont-tl.gif">
      <img class="ContainerTopRight" src="images\cont-tr.gif">
      <img class="ContainerBottomLeft" src="images\cont-bl.gif">
      <img class="ContainerBottomRight" src="images\cont-br.gif">
      <div>
            <a href="Design/eater.aspx">Databases</a><BR>
            <img id="mainContent_TreeView_btn0_0" onclick="ToggleDisplay(this, div0_0)" src="images/plus.gif" alt="" border="0" />
            <div id="mainContent_TreeView_div0_0" style="text-align:left;margin-left:0in;">
                  <a href="Design/someplace.aspx">PeCON</a><BR>
                  <img id="mainContent_TreeView_btn0_1" onclick="ToggleDisplay(this, div0_1)" src="images/plus.gif" alt="" border="0" />
                  <div id="mainContent_TreeView_div0_1" style="text-align:left;margin-left:0.2in;">
                        <a href="Design/blah.aspx">Stored Procedures</a><BR>
                        <a href="Design/blah.aspx">Tables</a><BR>
                        <a href="Design/blah.aspx">Views</a><BR>
                        <a href="Design/blah.aspx">User Defined Functions</a><BR>
                  </div>
            </div>
      </div>
</div>