Limit the number of child pages

I have this html structure:

<home>
 <Web>
   <web 1>
   <web 2>
   <web 3>
   <web 4>
   <web 5>
   <web 6>
   <web 7>
 </web>
 <Print>
   <Print 1>
   <Print 2>
   <Print 3>
   <Print 4>
   <Print 5>
 </print>
 <Art>
   <Art 1>
   <Art 2>
   <Art 3>
   <Art 4>
   <Art 5>
   <Art 6>
 </art>
</home>

I use this to display the grandchildren content while hiding its parent

<?php $counter = 1 ?>
<div class="row-fluid">

<?php
if ( have_posts() ) {
while ( have_posts() ) {
the_post();

$args=array(
        'orderby' => 'menu_order',
        'order' => 'ASC',
        'posts_per_page' => -1,
'post__not_in' => array(4,368,358,354),
        'post_type' => 'page',
        'post__in' => $pageIDs
);

$childpages = new WP_Query($args);

if($childpages->post_count > 0) { /* display the children content  */
    while ($childpages->have_posts()) {
         $childpages->the_post(); ?>
<div class="span4">
            <?php
        echo "<h2>".get_the_title()."</h2>";
                echo the_content();
    ?>
</div>
<? if ($counter % 3 == 0): ?>
</div>
<div class="row-fluid">
    <?php endif; ?>
<?php $counter++; ?>

   <?php }
}
wp_reset_query();
}
}

?>
</div>

At the moment all grandchildren are displayed, how do I limit the number of grandchildren to be 3 per type (3 for the print, 3 for the web and 3 for the art)?