How do I get child subcategories of custom taxanomy in dependant dropdown?

My categories structure like this:

Cat 1
subCat 1
subCat 2
anothSubCat 1
anothSubCat 1
Cat 2
subCat 1
subCat 2
Cat 3

The categories are from custom post type and the current code looks like this:

`function getall() {
?>

jQuery(document).ready(function () {
jQuery(‘#getall’).change(function () {
var mainCat = jQuery(‘#getall’).val();
// call ajax
jQuery.ajax({
url: “/wp-admin/admin-ajax.php”,
type: ‘POST’,
data: ‘action=category_select&parentID=’ + mainCat,
success: function (results)
{
jQuery(“#getsub”).html(results);
}
});
});
});

<form action="/” method=”get”>

Thats all, submit now !

<?php
}
die();
} // end if
}

add_action('wp_ajax_category_select', 'ajax_now');
add_action('wp_ajax_nopriv_category_select', 'ajax_now');`

I am able to get the parent categories list of the custom post type in the first drop-down. However, upon selecting the parent category, I am not getting the child category of the selected parent category in the dependent drop-down below.