LogicalTreeHelper.GetChildren does not find anything within ContentTemplate

Question

I have a ContentControl which tamplate is changed based on some value in DataContext.

<ContentControl x:Name="params_control">
    <ContentControl.Style>
        <Style TargetType="ContentControl">
            <Style.Triggers>
                <DataTrigger Binding="{Binding Type}" Value="Type1">
                    <Setter Property="Template" Value="{StaticResource template1}" />
                </DataTrigger>
                <DataTrigger Binding="{Binding Type}" Value="Type2">
                        <Setter Property="Template" Value="{StaticResource template2}" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </ContentControl.Style>
</ContentControl>

The templates are a grid with a number of TextBlocks and TextBoxes like this:

<ControlTemplate x:Key="template1" TargetType="{x:Type ContentControl}">
    <Grid>
        <TextBlock Text="Period"/>
        <TextBox Text="{Binding Period}"/>
    </Grid>
</ControlTemplate>
<ControlTemplate x:Key="template2" TargetType="{x:Type ContentControl}">
    <Grid>
        <TextBlock Text="Period2"/>
        <TextBox Text="{Binding Period2}"/>
    </Grid>
</ControlTemplate>

Template swithching and databinding works well.

The problem is that I need to loop through the input fields in the currently loaded template to check for validation errors but a call to LogicalTreeHelper.GetChildren(params_control) returns nothing.

Why does LogicalTreeHelper not see templated parent children?