JSFiddle - React, Tailwind, and code Playground

HTML

<div id="parent">
    <div id="childDiv"></div>
    
</div>

CSS

#parent{
    display:block;
    width: 100%;
    height: 300px;
    background: red;
    padding: 10px;
    box-sizing: border-box;
}
#childDiv{
       box-sizing: border-box;
       width: 100%; //or any percentage width you want
       padding: 50px;
    background:green;
    height: 100%;
}