Output on top of the page with add shortcode

Hi,

I am using this function but the output appears on top of the page right after #main is opened and not in the element I placed the shortcode.
What is wrong with the code?

function evenementen() {
$today = current_time('Ymd');
$args = array(
    'post_type' => 'evenementen',
    'posts_per_page' => '4',
    'meta_key'  => 'datum_evenement',
    'order' => 'DESC',
    'orderby' => 'meta_value',
    'meta_query' => array(
      array(
            'key'   => 'datum_evenement',
            'compare' => '>=',
            'value'   => $today,
        ),
    ),
);

$children = new WP_Query($args);
?>

<?php if ($children->have_posts()) : ?>
  <?php while ($children->have_posts()) : $children->the_post(); $fields = (object) get_fields(); ?>
    <div class="event row">
      <div class="event-logo col-sm-4">
        <a href="<?php the_permalink(); ?>"><?php the_post_thumbnail( 'large', array( 'class' => 'img-responsive' ) ); ?></a>
      </div>
      <div class="event-details col-sm-8">
        <h3 class="underline"><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
        <h5><?php the_field('datum_evenement'); ?></h5>
        <p class="call-to-action"><a href="<?php the_permalink(); ?>">Read More</a></p>
      </div>
    </div>
  <?php endwhile; wp_reset_postdata(); ?>
<?php endif; ?>
<?php

} add_shortcode('nm2d_evenementen', 'evenementen');