$args = array(
'post_type' => 'page',
'post_parent' => 'your_parent_page_ID',
);
$query = new WP_Query( $args );
if ( $query->have_posts() ) :
echo '<ul>';
while ( $query->have_posts() ) :
$query->the_post();
echo '<li>' . get_the_ID() . '</li>';
endwhile;
echo '</ul>';
endif;
wp_reset_postdata();
Put your parent page ID intead of your_parent_page_ID
and you will recieve list with child pages id.