Get WordPress Child Page IDs

$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.