My friend, just take a look at their codex here https://codex.wordpress.org/Function_Reference/get_footer and you will know how. In your case, no need to add these
require_once get_stylesheet_directory() . '/footer.php';
require_once get_stylesheet_directory() . '/footer-set.php';
require_once get_stylesheet_directory() . '/footer-tag.php';
You only need to repair your tt_get_footer like this
//Get Theme Specific Footer
function tt_get_footer() {
$footer = get_option('tt_footer_layout');
if($footer == 'tag') {
get_footer('tag');
} else if ($footer == 'set') {
get_footer('set');
} else {
get_footer(); // This one will use footer.php
}
}
Then you will get your child theme footer overriding