JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" action="">
    
    <input type="checkbox" name="key" />под ключ
    <input type="text" name="sum_key" value="1" /> шт.
    <br />
    <input type="checkbox" name="key" />под ключ
    <input type="text" name="sum_key" value="1" /> шт.
    <br />
    <input type="checkbox" name="key" />под ключ
    <input type="text" name="sum_key" value="1" /> шт.
    
</form>

CSS

input[name="sum_key"] {
    position: relative;
    background: #eee;
    text-indent: 100%;
    z-index: -1;
}

input[name="key"]:checked + input[name="sum_key"] {
    background: #fff;
    text-indent: 0;
    z-index: 1;
}