Nested docksites with MVVM

I have a MainView with a Docksite that contains a TabbedMDIHost and ToolWindow. This is bound using DocumentItemsSource to a ObservableCollection of Views in my MainViewModel, lets call them ObservableCollection<Child1View> Child1ViewCollection.

In MAINVIEW.XAML

<docking:DockSite x:Name="dockSite" DocumentItemsSource="{Binding Child1ViewCollection}">

       <docking:SplitContainer Orientation="Horizontal">

                <docking:ToolWindowContainer >
                    <docking:ToolWindow Title="Open Workspaces" DefaultDockSide="Left" Style="{StaticResource ToolWindowStyle}">
                        <...ToolWindowContents>

                    </docking:ToolWindow>
                </docking:ToolWindowContainer>

                <docking:Workspace>
                    <docking:TabbedMdiHost x:Name="TabbedMdiHost_Workspaces" TabStripPlacement="Top"/>
                </docking:Workspace>
            </docking:SplitContainer>
        </docking:DockSite>

Each Child1View.xaml then contains their own Docksite, bound in their DocumentItemsSource to an ObservableCollection<Child2View> Child2ViewCollection in Child1ViewModel.

In CHILDVIEW1.XAML

<docking:DockSite x:Name="dockSite_Child" DocumentItemsSource="{Binding Child2ViewCollection}">
                <docking:Workspace>
                <docking:TabbedMdiHost>
                        <docking:TabbedMdiContainer x:Name="TabbedMdiHost" TabStripPlacement="Top">
                        </docking:TabbedMdiContainer>
                </docking:TabbedMdiHost>
                </docking:Workspace>
            </docking:DockSite>

In my MainView the tab control populated with Child1View tabs works perfectly and fires WindowsRegistered / SourceChanged events when the collection is changed. But within the child view, although the Child2ViewCollection is initialised and populated, the views are never opened, no Window Registered / Requested or SourceChanged Events fire. If I explicitly add a document window in using XAML it does appear.

Any advice on where I am going wrong when the implementation of both docksites is the same?