Query to get child pages of current page and display it in action hook

I was making a beginner’s mistake. I was not defining the global variable post. I publish my complete code for the community. May be useful for something like me.

<?php

add_action( 'genesis_after_entry', 'capitulos_curso' );
function capitulos_curso(){
global $post;          
$args = array(
    'post_type'      => 'curso',
    'posts_per_page' => -1,
    'post_parent'    => $post->ID,
    'order'          => 'ASC',
    'orderby'        => 'menu_order'
 ); 

$parent = new WP_Query( $args );

?>

<?php if ( $parent->have_posts() ) : ?>
    <ul class="parent-page">    
        <?php while ( $parent->have_posts() ) : $parent->the_post(); ?>    
        <li><a href="https://wordpress.stackexchange.com/questions/307989/<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php the_title(); ?></a></li>
    </ul>
    <?php endwhile; ?>

<?php endif; wp_reset_postdata();} ?>
<?php
genesis();