JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="main-menu">
    <li><a href="#">Parent Link #1</a></li>
    <li><a href="#">Parent Link #2</a>
        <ul class="sub-menu">
            <li><a href="#">Child Link #1</a></li>
            <li><a href="#">Child Link #1</a></li>
        </ul>
    </li>
    <li><a href="#">Parent Link #3</a></li>
    <li><a href="#">Parent Link #4</a></li>
    <li><a href="#">Parent Link #5</a></li>
    <li><a href="#">Parent Link #666<a><li>
</ul>

JavaScript

$('.main-menu > li').find('ul.sub-menu').parent('li').append($('<div/>').addClass('expand'));