In this tutorial to show How to get child pages of specific parent page in sidebar dynamically

Ex : Products is a parent page and Product-1,product-2,etc are child page(sub-pages)

STEP 1 : To get child pages

Add the below code in functions.php 

function show_all_children($parent_id, $post_id, $current_level)
{ 
$top_parents = array();
$top_parents = get_post_ancestors($post_id);
$top_parents[] = $post_id;

$children = get_posts(
 array(
 'post_type' => 'page'
 , 'posts_per_page' => -1
 , 'post_parent' => $parent_id
 , 'order_by' => 'title'
 , 'order' => 'ASC'
));

if (empty($children)) return;

echo '<ul class=" sub-pages children level-'.$current_level.'-children">';

foreach ($children as $child)
{
if(is_page($child->ID ))
{
 $sctive="child-page-active"; //class of current(active) child page 
 
echo '<li class="p-4 mt-0 '.$sctive.'"';
}
else 
{ 
echo '<li class="p-4 mt-0" ';
}
 
echo '>';

echo '<span class="glyphicon glyphicon-triangle-right" aria-hidden="true"></span>
<a class="" href="'.get_permalink($child->ID).'">';
echo apply_filters('the_title', $child->post_title);
echo '</a>';

// now call the same function for child of this child
 if ($child->ID && (in_array($child->ID, $top_parents)))
 {
 show_all_children($child->ID, $post_id, $current_level+1);
 }

echo '</li>';
}

echo '</ul>';
}

STEP 2 : To display the list of child pages

Note : In this i’ve displayed the child pages title as list ,you can add div, section,etc instead of ‘<li>’  tag.

Add this code to where ever you want to display the lists.Add in sidebar ‘text widget’ to get the list in sidebar.

<h2>
<?php 
$current = $post->ID;
$parent = $post->post_parent;
$grandparent_get = get_post($parent);
$grandparent = $grandparent_get->post_parent;
?>

<?php 
if ($root_parent = get_the_title($grandparent) !== $root_parent = get_the_title($current)) 
{
echo get_the_title($grandparent); }else {echo get_the_title($parent); 
}
?>
</h2>

<?php
$parents_ids = get_post_ancestors($post->ID);
$top_parent_id = (count($parents_ids) > 0) ? $parents_ids[count($parents_ids)-1] : $post->ID;
show_all_children($top_parent_id, $post->ID, 1);
?>

Add the above code and check it . you can get the list of child pages titles in the sidebar.