Unable to include a file in child theme

get_stylesheet_directory_uri()

Note that this returns a properly-formed URI; in other words, it will be a web-address (starting with http:// or https:// for SSL). As such, it is most appropriately used for links, referencing additional stylesheets, or probably most commonly, images.

get_stylesheet_directory()

Retrieve stylesheet directory Path for the current theme/child theme.
Note: Does not contain a trailing slash.
Returns an absolute server path (eg: /home/user/public_html/wp-content/themes/my_theme), not a URI.

So I guess this

require_once( get_stylesheet_directory() . '/functions/taxonomy-images.php' );

should be fine.