WooCommerce in WordPress: add custom user meta data to order meta data on order completion (user should not see it)

Question

I have taken a couple of programming courses, but am not a programmer. 9 times out of 10, I can figure out how to modify code to suit my purposes. This time... I'm stuck.

I would like to add the user's date of birth to the order details and display it on the "Edit order" admin page. The date of birth should not be viewable by the user when they view the order while logged in, or in any emails that they receive about the order.

The date of birth is entered by the user during registration via a plugin called Custom User Registration Fields for WooCommerce. The user meta key that contains the date of birth is afreg_additional_97423.

I tried adapting the code from this page: WooCommerce: Add custom meta as hidden order item meta for internal use

And ended up with this (added via Code Snippets plugin rather than to functions.php)

Snippet 1:

// Add date of birth to orders
add_action('woocommerce_checkout_create_order_line_item', 'add_custom_hidden_order_item_meta_data', 20, 4 );
function add_custom_hidden_order_item_meta_data( $item, $cart_item_key, $values, $order ) {

    // Set user 'afreg_additional_97423' custom field as admin order item meta (hidden from customer)
    if( $meta_value = get_user_meta( $order->get_user_id(), 'afreg_additional_97423', true ) )
        $item->update_meta_data( '_afreg_additional_97423', $meta_value );
}

Snippet 2:

// create clean label name for meta key afreg_additional_97423 on admin order items
add_filter('woocommerce_order_item_display_meta_key', 'filter_wc_order_item_display_meta_key', 20, 3 );
function filter_wc_order_item_display_meta_key( $display_key, $meta, $item ) {

    // Set user meta custom field as order item meta
    if( $meta->key === '_afreg_additional_97423' && is_admin() )
        $display_key = __("date of birth", "woocommerce" );

    return $display_key;    
}

But I do not see the date of birth on the admin order page, and I don't think it even got added to the order.

Am I close?

Also... do they need to be separate snippets?