Mark Tree node expandable

   
    
<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      HorizontalAlignment="Center" VerticalAlignment="Center">

<TreeView>
  <TreeViewItem Header="A" IsExpanded="True">
    <TreeViewItem Header="B" />
    <TreeViewItem Header="C" IsExpanded="True">
      <TreeViewItem Header="D" />
      <TreeViewItem Header="E" />
    </TreeViewItem>
    <TreeViewItem Header="F" />
  </TreeViewItem>
  <TreeViewItem IsExpanded="True">
    <TreeViewItem.Header>
      <StackPanel Orientation="Horizontal">
        <Ellipse Fill="Blue" Width="15" Height="15" />
        <TextBlock Text="Third top-level item" />
        <Ellipse Fill="Blue" Width="15" Height="15" />
      </StackPanel>
    </TreeViewItem.Header>
    <TreeViewItem Header="Child a" />
    <TreeViewItem Header="Child b" />
    <TreeViewItem Header="Child c" />
  </TreeViewItem>
</TreeView>
</Page>