Can’t override footer.php in child theme

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

tech