JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="wrapper">
        <span class="child1">Dynamic Width</span>
    </div>
    <span class="child2">Fixed</span>
</div>

<div class="container">
    <div class="wrapper">
        <span class="child1">Here is a Dynamic Width box</span>
    </div>
    <span class="child2">Fixed</span>
</div>

CSS

.container {
    width: 200px;
    height: 50px;
    border: 1px solid green;
    
    display: table;
    table-layout: fixed;
    transition: all 2s;
}
span {
   height: 50px;
   display: table-cell;
    transition: all .2s;
}
.child1 {
    background: tomato;
    width: 100%;
}
.child2 {
    background: aqua;
    width: 0px;
    overflow: hidden;
    transition: all .2s;
}
.container:hover .child2 {
    width: 50px;
}