php, js - echoing a number based on how many children there are?

Creating a site in with FrogCMS and using alot of jquery to create an all in one screen site.

having trouble in php echoing a number for the number of children for a subject in the database which i can then use as a var in js.

some things ive tried, but im not really a developer so having issues

here is some code frog supplies to show to utilise articles for example, and articles then has childen in the database.

<?php $page_article = $this->find('/articl开发者_开发技巧es/'); ?>
<?php $last_article = $page_article->children(array('limit' => 1, 'order' => 'page.created_on DESC')); ?>

but im not sure how to create a discrete number to use with the jquery concerning the number of children??

thanks

May this could work :

<script type="text/javascript">
var nbChildren = <?php $comments = $page_article->children(); echo sizeof($comments); ?>
</script>