MainWindow.xaml

<dx:DXWindow
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
        xmlns:dxb="http://schemas.devexpress.com/winfx/2008/xaml/bars"
        xmlns:dxe="http://schemas.devexpress.com/winfx/2008/xaml/editors"
        xmlns:dxa="http://schemas.devexpress.com/winfx/2008/xaml/accordion" 
        xmlns:local="clr-namespace:ChildrenSelector" 
        x:Class="ChildrenSelector.MainWindow"
        Title="MainWindow" Height="350" Width="525">
    <dx:DXWindow.Resources>
        <local:MySelector x:Key="mySelector"/>
    </dx:DXWindow.Resources>
    <dx:DXWindow.DataContext>
        <local:ViewModel/>
    </dx:DXWindow.DataContext>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="1*"/>
            <ColumnDefinition Width="2*"/>
        </Grid.ColumnDefinitions>
        <Border Margin="5" Grid.Column="0" BorderBrush="Black" BorderThickness="1">
            <dxa:AccordionControl Name="accordion" SelectionUnit="SubItem" ItemsSource="{Binding MyData.Categories }" 
                                  SelectedItem="{Binding SelectedItem}" ChildrenSelector="{StaticResource mySelector}">
                <dxa:AccordionControl.Resources>
                    <DataTemplate DataType="{x:Type local:Category}">
                        <TextBlock Text="{Binding CategoryName}"/>
                    </DataTemplate>
                    <DataTemplate DataType="{x:Type local:Item}">
                        <TextBlock Text="{Binding ItemName}"/>
                    </DataTemplate>
                </dxa:AccordionControl.Resources>
            </dxa:AccordionControl>
        </Border>
        <Border Margin="5" Grid.Column="1" BorderBrush="Black" BorderThickness="1">
            <StackPanel>
                <StackPanel Orientation="Horizontal">
                    <Label Margin="5" VerticalAlignment="Center">Item Name</Label>
                    <dxe:TextEdit Margin="5" Width="150" Text="{Binding SelectedItem.Description}"/>
                </StackPanel>
                <StackPanel Orientation="Horizontal">
                    <Button Margin="5" Content="Expand All Items" 
                            Command="{Binding ElementName=accordion, Path=Commands.ExpandAllItems}" />
                    <Button Margin="5" Content="Collapse All Items" 
                            Command="{Binding ElementName=accordion, Path=Commands.CollapseAllItems}" />
                </StackPanel>
            </StackPanel>
        </Border>
    </Grid>
</dx:DXWindow>