title: Silverlight GetTemplateChild of control within control returns null
body: <p>I'm trying to add an indicator (I'm using TextBlock) to the datepicker control.</p>
  
  <p>Visually it works but I can't get the control via GetTemplateChild. I assume it's something to do with the fact that the TextBlock control I added is in the DatePickerTextBox style template as opposed to the DatePicker style template.</p>
  
  <p>I've tried DefaultStyleKey (although I don't think this makes sense as it's the TextBox control within DatePicker that's the problem) and using OnApplyTemplate and UpdateLayout on the TextBox control.</p>
  
  <p>Here's a snippet of the Dictionary.xaml</p>
  
  <pre><code>&lt;Style x:Key="Ind_DatePickerTextBoxStyle" TargetType="primitives:DatePickerTextBox"&gt;
     ...
     &lt;Grid VerticalAlignment="Stretch"&gt;
        &lt;TextBlock x:Name="Indicator" Text="*" Style="{StaticResource IndicatorStyle}" Visibility="Collapsed"/&gt;
  
  ...
  
  &lt;!--datepicker style snippet--&gt;
  
  &lt;primitives:BF_DatePickerTextBox 
     x:Name="TextBox" 
     SelectionBackground="{TemplateBinding SelectionBackground}" 
     Background="{TemplateBinding Background}" 
     BorderBrush="{TemplateBinding BorderBrush}" 
     BorderThickness="{TemplateBinding BorderThickness}" 
     Padding="{TemplateBinding Padding}" 
     Grid.Column="0" 
     Style="{StaticResource Ind_DatePickerTextBoxStyle}" /&gt;
  </code></pre>