JSFiddle - React, Tailwind, and code Playground

HTML

<div class='container child-2'>
    <div class='child-1'>1</div>
    <div class='child-2'>2</div>
    <div class='child-3'>3</div>
</div>

CSS

.container > div { display:none}

JavaScript

var id = $('.container').attr('class').replace('container child-','');
createCSSClass('.container > .child-' + id, 'display:block !important');


function createCSSClass(selector, style)
{ 
    
 // using information found at: http://www.quirksmode.org/dom/w3c_css.html
 // doesn't work in older versions of Opera (< 9) due to lack of styleSheets support

 if(!document.styleSheets) return;
 if(document.getElementsByTagName("head").length == 0) return;
     
 var stylesheet;
 var mediaType;
 if(document.styleSheets.length > 0)
 {
  for(i = 0; i<document.styleSheets.length; i++)
  {
   if(document.styleSheets[i].disabled) continue;
   var media = document.styleSheets[i].media;
   mediaType = typeof media;
   // IE
   if(mediaType == "string")
   {
    if(media == "" || media.indexOf("screen") != -1)
    {
     styleSheet = document.styleSheets[i];
    }
   }
   else if(mediaType == "object")
   {
    if(media.mediaText == "" || media.mediaText.indexOf("screen") != -1)
    {
     styleSheet = document.styleSheets[i];
    }
   }
   // stylesheet found, so break out of loop
   if(typeof styleSheet != "undefined") break;
  }
 }
 // if no style sheet is found
 if(typeof styleSheet == "undefined")
 {
  // create a new style sheet
  var styleSheetElement = document.createElement("style");
  styleSheetElement.type = "text/css";
  // add to <head>
  document.getElementsByTagName("head")[0].appendChild(styleSheetElement);
  // select it
  for(i = 0; i<document.styleSheets.length; i++)
  {
   if(document.styleSheets[i].disabled) continue;
   styleSheet = document.styleSheets[i];
  }
  // get media type
  var media = styleSheet.media;
  mediaType = typeof media;
 }
 // IE
 if(mediaType == "string")
 {
  for(i = 0;i<styleSheet.rules.length;i++)
  {
   // if there is an existing rule set up, replace it
   if(styleSheet.rules[i].selectorText.toLowerCase() == selector.toLowerCase())
   {
    styleSheet.rules[i].style.cssText = style;
    return;
   }
  }
  // or add a new rule
 ...