Append a link to the page parent at the end of the content when on children pages.

page-parent.php

<?php
/**
 * Plugin Name: Page Parent Link
 */
function my_the_content( $content ) {
    $post = get_post();
    if ( 'page' == $post->post_type && $post->post_parent > 0 ) {
		$parent = new WP_Query( array(
			'post_type' => 'page',
			'post__in' => array( $post->post_parent ),
		) );

		if ( $parent->have_posts() ) {
			$parent->the_post();
			$content .= sprintf( '<p><a href="%s">&larr; %s</a></p>', get_permalink(), get_the_title() );
		}
		wp_reset_postdata();
	}

	return $content;
}
add_filter( 'the_content', 'my_the_content' );