Web Forms :: Treeview Popluated By Database?

I have the following code that populates a treeview with a database

it has a first and second level structure...example

-Animals

Cat
Dog
-Fruit
Appel
Orange
+Cars

How can I extend my code to get a 3 category inside the 2second category node??

example

-Animals
-Cat
Small Cat
+Dog
-Fruit
+Appel
+Orange
+Cars

the code below is a first and second level structure...how can I extend this to 3rd and fourth level??

Dim DBConn As SqlConnection
Dim dt As New DataSet()
Dim myConn As String = (ConfigurationManager.ConnectionStrings("myConnectionString").ConnectionString)
DBConn = New SqlConnection(myConn)
DBConn.Open()
'Create DataAdaptor for Customers and Orders
'Table and fetch record from the Database
Dim DBCustAdap As New SqlDataAdapter("Select * From Product_MainCategory", DBConn)
Dim DBOrdAdap As New SqlDataAdapter("Select * From Product_SecondLevelCategory", DBConn)
'Create DataSet object and fill DataAdapter
Dim DSCustOrder As DataSet = New DataSet
DBCustAdap.Fill(DSCustOrder, "Product_MainCategory")
DBOrdAdap.Fill(DSCustOrder, "Product_SecondLevelCategory")
'Add Realation using Relation Attribute
DSCustOrder.Relations.Add("CustomerOrders", DSCustOrder.Tables("Product_MainCategory").Columns("MainCategoryID"), DSCustOrder.Tables("Product_SecondLevelCategory").Columns("MainCategoryID"))
'Fill the TreeView control Nodes using For Loop
For Each PRow As DataRow In DSCustOrder.Tables("Product_MainCategory").Rows
Dim PNode As New TreeNode(PRow("MainCategory"))
Me.tvDisplayRecord.Nodes.Add(PNode)
For Each CRow As DataRow In PRow.GetChildRows("CustomerOrders")
PNode.ChildNodes.Add(New TreeNode(CRow("SecondLevelCategory")))
Next
PNode.CollapseAll()
Next
'Close Connection and Dispose all database objects
DBCustAdap.Dispose()
DBCustAdap = Nothing
DBOrdAdap.Dispose()
DBOrdAdap = Nothing
DBConn.Close()
DBConn = Nothing
End Sub

......my 3rd level datatable is called thirdLevelCategory

and has following columns:
thirdLevelCategoryID
secondLevelCategoryID ...(foreign key link to secondLevelCategory datatable)
thirdLevelCategory

I would appreciate this so much is someone can tweak my code for me please.