Add a custom header to a WPF Treeview

I have a treeview control in a WPF app that has 3 levels to it - Root, Child and Leaf. Something like:

<DataTemplate DataType="{x:Type local:Leaf}">
    <Grid>
        <StackPanel Orientation="Horizontal">                     
            <CheckBox IsChecked="True" Content="LeafCheckbox" />                     
            <TextBlock Text="{Binding Path=SomeValue}"/>                 
        </StackPanel>             
    </Grid>         
</DataTemplate>
<HierarchicalDataTemplate DataType="{x:Type local:Child}" ItemsSource="{Binding Children}">             
    <StackPanel Orientation="Horizontal">                 
        <TextBlock Text="Child " />                 
        <TextBlock Text="{Binding Path=SomeValue}" />             
    </StackPanel>         
</HierarchicalDataTemplate>
<HierarchicalDataTemplate DataType="{x:Type local:Root}" ItemsSource="{Binding Children}">             
    <Grid>                 
        <StackPanel Orientation="Horizontal">                     
            <TextBlock Text="Root " />                     
            <TextBlock Text="{Binding Path=SomeVa开发者_C百科lue}" />                 
        </StackPanel>            
    </Grid>        
</HierarchicalDataTemplate> 

My design requirement has a custom header to be able to Uncheck all, like so:

After fighting this a bit, I tried adding a leaf to the first index of the bounded list with a unique name, and in the XAML treat it and skin in differently. That kind of worked, but it screwed the lists and it just became too hairy to deal with - the definition of an ugly hack. I also thought about changing the actual HierarchicalDataTemplate, having it use a grid of 2 Rows, the bottom row being that special header - that worked but the problem was that the expander would always be vertically centered and not aligned with the "Root".

Any ideas how to nicely do this?