JSFiddle - React, Tailwind, and code Playground

HTML

<div class="some-class">
        <div>
             <p>The background of this should be red.</p>
        </div>
    </div>
    
    <div class="some-class">
        <div>
             <p>The background of this should be blue.</p>
        </div>
        <div>
             <p>The background of this should be blue</p>
        </div>
    </div>

CSS

.some-class div:only-child
    {
        background: red;
    }
    .some-class div:not(:only-of-type)
    {
        background: blue;
    }