How to accomplish a child page navigation?

I’m outlining a solution for you, it uses get_children to determine if there are any, then conditionally sets up the post id $p_id for the child_of parameter, so that the list of child pages from a certain parent always can be shown, by making use of wp_list_pages().

Code:

function wpse125273_child_page_nav() {
    global $post;

    $args = array( 
        'post_parent' => $post->ID,
        'post_type'   => 'page',
        'post_status' => 'publish'
    );
    $children = get_children( $args );

    if ( empty( $children ) ) {
        $p_id = $post->post_parent;
    } else {
        $p_id = $post->ID;
    }

    $args = array(
        'child_of' => $p_id,
        'post_type'   => 'page',
        'post_status' => 'publish'
    );
    wp_list_pages( $args );
}

Note: exemplary and untested