How to enqueue second stylesheet in Wordpress child theme?

I'm using a child theme and need to enqueue the stylesheet for a slider. The css file is located in a folder in the parent theme directory, i.e. /parent-theme/css/flexislider.css

I created the child theme using a plugin which added a functions.php file to the child theme directory with the following code:

<?php
add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
function theme_enqueue_styles() {
wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
wp_enqueue_style( 'child-style',get_stylesheet_directory_uri() . '/style.css',array('parent-style')
);
}

I assume I have to add another entry to the above function referencing flexislider.css but I'm not quite sure how to do it.

UPDATED

Based on what you added to the question, you should be able to just add to that function and register the stylesheet. The full function would look like this:

add_action('wp_enqueue_scripts', 'theme_enqueue_styles');
function theme_enqueue_styles() {
    wp_enqueue_style('parent-style', get_template_directory_uri() . '/style.css');
    wp_enqueue_style('child-style', get_stylesheet_directory_uri() . '/style.css',array('parent-style'));
    wp_enqueue_style('flex-slider', get_template_directory_uri() . '/css/flexislider.css');
}

I don't have much experience with child-themes, but following the model in that function, I think get_template_directory points to the parent theme and not the child theme, which you said the flexislider code is located.