trying to create a drop-down menu where the items are the children pages of a given parent page.
The list has an autosubmit script.

I’d like to add some css rule but unable to do so.
Moreover, I see the following warning appearing briefly everytime I do edit the page where I test the code:

Cannot modify header information – headers already sent by (output started at /etc etc)

Many thanks in advance

Here’s the code, not mine really: source

function dropdownbox_autosubmit() {
    wp_dropdown_pages(array('child_of' => 42,'show_option_none=Select Page')); ?>
    <script type="text/javascript">
        var pageDropdown = document.getElementById("page_id");
        pageDropdown.onchange = onPageSelect;
        function onPageSelect() {
            if ( pageDropdown.options[pageDropdown.selectedIndex].value > 0 ) {
                location.href = "<?php echo get_option('home');?>/post.php?page_id="+pageDropdown.options[pageDropdown.selectedIndex].value;
             }
         }
    </script>
<?php

}
add_shortcode( 'call_dropdownbox_autosubmit', 'dropdownbox_autosubmit' );