OK. I got this working. Here is my code for reference.
<?php
$tasks = get_posts(array(
'post_type' => 'task',
'meta_query' => array(
array(
'key' => 'task_parents', // name of custom field
'value' => '"' . get_the_ID() . '"', // matches exaclty "123", not just 123. This prevents a match for "1234"
'compare' => 'LIKE'
)
)
));
?>
<?php if( $tasks ): ?>
<table>
<?php foreach( $tasks as $tasks ):?>
<tr>
<td>
<a href="<?php echo get_permalink( $tasks->ID ); ?>">
<?php echo get_the_title( $tasks->ID ); ?>
</a>
</td>
</tr>
<?php endforeach; ?>
</table>
<?php endif; ?>
