Vista - Stretching a middle child control in WPF

In a row of a vertically-oriented StackPanel, I want a label, then a text
box, and then a checkbox. I want the label and checkbox as wide as their
text, and the text box to stretch based on the width of the parent (so that
the checkbox is docked to the right). How do I do this?

I tried the following (as a child to a vertically-oriented stack panel):

<DockPanel FlowDirection="RightToLeft" Height="Auto" Width="Auto">
<CheckBox FlowDirection="LeftToRight" Height="16"
HorizontalAlignment="Left" Width="Auto">CheckBox</CheckBox>
<DockPanel FlowDirection="LeftToRight" Height="Auto" Width="Auto">
<Label Height="28" HorizontalAlignment="Left" Width="Auto">Label</Label>
<TextBox Height="23" Width="Auto" />
</DockPanel>
</DockPanel>

Because of the RightToLeft flow direction of the outer dock panel, the first
child (the check box) appears to the right of the label and text box, and
because the last child (the dock panel holding the label and text box) is
Stretch and Auto width, the result is visually what I want. However, the tab
order goes to the check box before the text box.

How can I get the desired result?

Thanks.