You should use the pagename
parameter for WP_Query along with post_parent
. Limit your query to one post using posts_per_page
.
$parent_id = get_the_ID();
$args = array(
'post_type' => 'page'
'pagename' => 'page-2',
'post_parent' => $parent_id,
'posts_per_page' => 1
);
$posts = get_posts( $args );
$child = isset( $posts[0] ) ? $posts[0] : false;
if( $child ){
//Do Something
}
This, in theory, should render your desired post by slug using the parent ID.