Example of how to use WordPress' wp_localize_script() script to provide number of page children to a jQuery script

functions.php

<?php
// Your other your-theme/functions.php code goes here

add_action( 'wp_enqueue_scripts', 'yoursite_enqueue_scripts' );

function yoursite_enqueue_scripts() {
	global $post;

	if ( ! empty( $post ) && is_page( $post ) ) {
		wp_enqueue_script( 'yoursite-js', get_stylesheet_directory_uri() . '/js/yoursite.js');
		wp_localize_script( 'yoursite-js', 'YourSiteVar', array(
			'childCount' => count( get_children( $post->ID ) ),
		) );
	}
}

yoursite.js

// {theme_dir}/js/yoursite.js
(function($) {

    $(document).ready(function(){
        // Inspired by https://www.youtube.com/watch?v=PDd8shcLvHI
        if ( 0 == YourSiteVar.childCount ) {
            alert('Yes, we have NO CHILDREN on this page today!');
        } else {
            alert('No, we have %d CHILDREN on this page today!'.replace(/%d/g,YourSiteVar.childCount) );
        }
    });

})(jQuery); // Fully reference jQuery after this point.