"next page" of parent in infinite loop

I'm trying to include a "Next Service" button on a website that hopefully take you to the next page under the Services page (Parent) so no matter which service you are on, it can loop through to the next (regardless if it's the first or last page).

However the code I have does that but when it gets to the last one, it carries on to the next page - regardless of the fact it's not under 'Services'.

?php 
    $nextPost = get_next_post();
    if($nextPost) {
        $nextPostID = $nextPost-ID;
?
    a href="?php echo get_permalink( $nextPostID ); ?" 
        ID ?php echo get_the_id( $nextPostID ); ? - ?php echo get_the_title( $nextPostID ); ?
    /a
?php } else {
        $first_post = get_posts( array(
            'post_type' = 'page',
            'posts_per_page'   = 1,
            'order' = 'ASC'
        ) );
        ?
        a href="?php echo get_permalink( $first_post[0]-ID ); ?" 
            First: ?php echo get_the_title( $nextPostID ); ?
        /a
?php } ?
?php wp_reset_postdata(); ?