Hey Mike, thank you so much for your help! I used the snippet that you posted and actually tweaked it a little to fit my situation better. Since I wanted to hide the subpages of one specific page of the Page Sidebar navigation (not my main navigation menu), I used:

function custom_script(){
  ?>
  <script>
(function($){
  $(document).ready(function(){ 
      $('.nested_nav > li.page-item-31276').find('ul').hide();
    });  
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'custom_script');

Thank you again!