JSFiddle - React, Tailwind, and code Playground

HTML

<div id="menu">
    <ul>
        <li class="active">
            <h3 id="p1">Parent 1</h3>
            <ul>
                <li><a href="#">Child 1</a>
                </li>
                <li><a href="#">Child 2</a>
                </li>
                <li><a href="#">Child 3</a>
                </li>
                <li><a href="#">Child 4</a>
                </li>
            </ul>
        </li>
    </ul>
</div>

CSS

* {
    margin: 0;
    padding: 0;
}
body {
    background: #440001;
    font-family: arial, verdana;
}
#menu {
    background: #004050;
    width: 250px;
    margin: 30px auto 0 auto;
    color: white;
}
#menu h3 {
    font-size: 12px;
    line-height: 34px;
    padding: 0 10px;
    cursor: pointer;
    background: #6bba70;
    background: linear-gradient(to bottom, #a90329 0%, #8f0222 44%, #6d0019 100%);
}
#menu h3:hover {
    text-shadow: 0 0 1px rgba(255, 255, 255, 0.7);
}
#menu li {
    list-style-type: none;
}
#menu ul ul li a {
    color: white;
    text-decoration: none;
    font-size: 11px;
    line-height: 27px;
    display: block;
    padding: 0 15px;
    transition: all 0.15s;
}
#menu ul ul li a:hover {
    background: #003545;
    border-left: 5px solid lightgreen;
}
#menu ul ul {
    display: none;
}
#menu li.active ul {
    display: block;
}

JavaScript

$('#menu li ul li').each(function(i, e){
    $(this).append('<span class="arr"></span>');
    $(".arr").attr("id", "id_" + i);
});