JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<link rel="stylesheet" href="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<script src="//stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">
<div class="container mt-5">

  <div class="row p-1 bg-primary" id="parent">
    <div class="col">
    
      <div class="row mb-1 bg-secondary">
        <div class="col">
          I'd like this to be able to be whatever height based on the content
        </div>
      </div>
      
      <div class="row child bg-secondary h-100">
        <div class="col">
          I want this to just take up the remaining height of the parent row
        </div>
      </div>
    </div>
  </div>
  
</div>

CSS

#parent {
  height: 300px;
}