functions.php

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

$child_query = new WP_Query( $args );

if ( $child_query->have_posts() ) {
	echo '<ul>';
	while ( $child_query->have_posts() ) {
		$child_query->the_post();
		echo '<li>' . get_the_title() . '</li>';
	}
	echo '</ul>';
} else {
	// no posts found
}

wp_reset_postdata();