@SELECT current ID?

@SELECT `pagetitle`,`id` FROM `[[+PREFIX]]site_content` WHERE `parent` = $_REQUEST['id'] AND `published` = 1

@SELECT `pagetitle`,`id` FROM `[[+PREFIX]]site_content` WHERE `parent` = [[+id]] AND `published` = 1

@SELECT `pagetitle`,`id` FROM `[[+PREFIX]]site_content` WHERE `parent` = [[*id]] AND `published` = 1

@SELECT `pagetitle`,`id` FROM `[[+PREFIX]]site_content` WHERE `parent` = 90 AND `published` = 1

[[!ShowChildren]]

<?php
/* ShowChildren snippet */

$children = $modx->resource->getMany('Children');

$output = "";

if (! empty ($children)) {
    $output = '<ul>';
    foreach $children as $child) {
        $output .= '<li>' . $child->get('pagetitle') . '</li>'; 
    }
    $output = '</ul>';
} 

return $output;

<?php
$children = $modx->resource->getMany('Children');
  $output = '';
  
  $list = array();
  foreach($children as $child)
  {
//check if correct template
    if($child->get('published')&&$child->get('template')==21)
    {
      $list[]  = $child->get('pagetitle').'=='.$child->get('id');
    }
  }
  
  $output .= implode('||',$list);
  
  return $output;