Is it possible to add a child post into the parent page?

First, you need to query the child Pages:

$pageargs = array(
  'numberposts' => -1,
  'orderby' => 'menu_order',
  'order' => 'ASC',
  'post_type' => 'page',
  'post_parent' => $post->ID
);
$childpages = get_posts( $pageargs );

Then, you need to create your custom Loop:

foreach ( $childpages as $childpage ) {
    // Output whatever you want here, such as:
    ?>
    <h2><?php echo $childpage->post_title; ?></h2>
    <?php echo $childpage->post_exerpt; ?>
<?php }

Note, however, that Pages do not by default support excerpts. So, you’ll need to enable that, in functions.php:

// Add Excerpt to Pages
add_post_type_support( 'page', 'excerpt' );

Then, you should be all set.