Hi there,
Thanks for writing in! To achieve this, please add the following jQuery script in your Customizer via Appearance > Customize > Custom > Edit Global Javascript
jQuery(document).ready(function($){
$('.menu-item-has-children').click(function(){
$(this).find('.sub-menu').toggle();
});
});
Don’t forget to clear your browser’s cache after adding the code. Let us know how this goes!