Use the HasChilds property to display expanding/collapsing buttons for a node to build your virtual tree. The property has no effect if the node contains already visible child nodes. Use the BeforeExpandNode event to notify your application that the user is about to expand or collapse a node. Use the Expanded property to expand or collapse a node. You can use the BeforeExpandNode event to cancel expanding specified nodes.

The following sample adds new child nodes to the node that's about to be expanded:

Private Sub XMLGrid1_BeforeExpandNode(ByVal Node As EXMLGRIDLibCtl.INode, Cancel As Variant)
    If Not Node.Expanded Then
        With Node.Nodes
            With .Add("New Node")
                .HasChilds = True
            End With
        End With
    End If
End Sub