WordPress: Menu for Child pages (Split Menu)

splitmenu.php

<?php
function mwe_get_parent_title() {
    $post = get_post();
    return get_the_title($post->post_parent);
}

function mwe_get_sidebar_menu() {
    $post = get_post();
    if ( is_page($post->ID) && $post->post_parent ) {
        $children = get_pages(array(
            'child_of' => $post->post_parent,
            'sort_column' => 'menu_order',
            )
        );
    } else {
        $children = get_pages(array(
            'child_of' => $post->ID,
            'sort_column' => 'menu_order',
            )
        );
    }
    
    ?>
    <ul class="submenu">
    <?php
    foreach ($children as $child) {
        $is_active = ($child->ID === get_the_ID()) ? 'active' : '';
        ?>
        <li class="submenu__item"><a class="submenu__link <?= $is_active ?>" href="<?= get_permalink($child->ID) ?>">
            <?= $child->post_title ?>
        </a></li>
    <?php } ?>
    </ul>
    <?php
}