Delete all children when parent is deleted – frontend

Well, interesting question. As far I’ve researched the hooks are called on deleting post passes the deleted post ID as parameter. At the time of deleting a parent page it works well but when you hooked a function on trashed_post to delete the child page it won’t work cause every time it will get the trashed post id and here it’ll be the ID of recently deleted child page. So it will not work as expected.

Now come to the solution. Here I’ve rewritten your functions with a SQL command to change the post status of the child pages to trash–

function delete_post(){
    global $post;
    $deletepostlink= add_query_arg( 'frontend', 'true',   get_delete_post_link( get_the_ID() ) );
    if (current_user_can('edit_post', $post->ID)) {
        echo '<span><a class="post-delete-link" onclick="return  confirm(\'Are you sure to delete?\')" href="' . $deletepostlink . '">Delete this </a></span>';
    }
}

//Redirect after delete post in frontend
add_action('trashed_post','trash_redirection_frontend');
function trash_redirection_frontend($post_id) {
    if ( filter_input( INPUT_GET, 'frontend', FILTER_VALIDATE_BOOLEAN ) ) {
        $args = array(
            'posts_per_page' => -1,
            'order'=> 'ASC',
            'post_parent' => $post_id,
            'post_type' => 'page'
        );
        // Filter through all pages and find Portfolio's children
        $children = get_children( $args );
        global $wpdb;
        foreach($children as $child){
            $childs[$child->ID] = $child->ID;
        }
        $sql = "UPDATE {$wpdb->posts} SET post_status="trash" WHERE ID IN (" . implode( ', ', $childs ) . ")";
        $wpdb->query($sql);
        wp_redirect( get_option('siteurl')."https://wordpress.stackexchange.com/" );
        exit;
    }
}

Hope it’ll help you.