Using recursion to load controls into a Form

I have a process which uses recursion to generate controls from XML.  The system is complicated.  I've broken down as small as I can.  The last Plugin is visible, the rest are not.  I suspect `GenerateControls()` is broken.  Why don't all the plugins display?

//Form:

    public partial class PdLoadingForm : Form
    {        
        public PdLoadingForm()
        {
            InitializeComponent();

            PhysDocDocument document = MockDocument();//Generate some mock data
            GenerateControls(document.Nodes);//Generate controls using recursion.
        }

        private PhysDocDocument MockDocument()
        {
            PhysDocDocument document = new PhysDocDocument();
            PhysDocNode outerNode = new PhysDocNode();
            outerNode.Display = "outer Node";

            for(int i = 0; i < 3; i++)
            {
                PhysDocNode innerNode = new PhysDocNode() { Display = "test" + i };

                PhysDocNode innerNodeContent = new PhysDocNode() { Display = "IO" };
                innerNodeContent.Plugins.Add(new Plugin());
                innerNode.Nodes.Add(innerNodeContent);
                outerNode.Nodes.Add(innerNode);
            }

            document.Nodes.Add(outerNode);

            return document;

        }

        private void GenerateControls(List<PhysDocNode> children, CustomControl parent = null)
        {
            foreach (PhysDocNode node in children)
            {
                CustomControl parentControl = new CustomControl(node);                

                if (node.Nodes != null && node.Nodes.Count > 0)
                    GenerateControls(children: node.Nodes, parent: parentControl);

                foreach (Plugin plugin in node.Plugins)
                {  
                    Control childControl = plugin.CreateUIControl();//Ask the plugin for a control
                    AddControl(childControl: childControl, parent: parentControl);
                }

                AddControl(childControl: parentControl, parent: parent);
            }
        }

        private void AddControl(Control childControl, Control parent)
        {
            childControl.Dock = DockStyle.Top;

            if(parent == null)//add to form
                Controls.Add(childControl);
            else//add to parent
                parent.Controls.Add(childControl);
        }
    }

//Plugin:

    public class Plugin
    {
        [XmlAttribute]
        public string type { get; set; }

        public Control CreateUIControl()
        {
            TextBox testBox = new TextBox();
            testBox.Text = "plugin";

            return testBox;
        }
    }

//CustomControl:

    public class CustomControl: UserControl
    {    
        public CustomControl(PhysDocNode nodeInfo)
        {
            InitializeComponent();
            Label label = new Label();
            label.Text = "Rtb..." + nodeInfo.Display;
            label.Dock = DockStyle.Top;
            contentPanel.Controls.Add(label);//just drop a panel on the user control in design view
        }