WordPress Side Navigation with Parent Heading and Child Sub Pages

Change the depth parameter to how many levels you want the walker to traverse.

From the docs depth parameter description.

Number of levels in the hierarchy of pages to include in the generated list. Accepts -1 (any depth), 0 (all pages), 1 (top-level pages only), and n (pages to the given n depth). Default 0.

Let us say 2 as the depth level , it outputs both SubPages and it’s first level children.

<?php
    global $post;
    $current_page_parent = ( $post->post_parent ? $post->post_parent : $post->ID );      
    wp_list_pages( array(
           'title_li' => '',
           'child_of' => $current_page_parent,
           'depth' => '2' // Traverses SubPage and it's first level children
         )
    );