WPF MDI maximize box

I'm using the WPF Multiple Document Interface from this site http://ift.tt/1ag4XHv. How can I maximize the box inside the code the same way it is done when I click maximize button?

        <DockPanel>
            <mdi:MdiContainer Theme="Luna" DockPanel.Dock="Top" Margin="0 20 0 0" Name="MainMdiContainer">
                <mdi:MdiChild Name="TestWindow"  Title="Child Window" Background="AliceBlue" Resizable="True" />
            </mdi:MdiContainer>
        </DockPanel>

I've tried something like

TestWindow.WindowState = System.Windows.WindowState.Maximized;

but when I do so I cannot see the buttons I normally see when I press the Maximize button.