JSFiddle - React, Tailwind, and code Playground

HTML

<div id="content-body" class="clearfix">

        <!-- detail div no 1 = 3 items inside -->
            <span class="countDiv0"></span> 
            <div class="detail">
                <div class="box">
                    Div item 1
                </div>
                <div class="box">
                    Div item 2
                </div>
                <div class="box">
                    Div item 3
                </div>
            </div>
            <br /><br />
            <!-- detail div no 1 = 4 items inside -->
            <span class="countDiv1"></span> 
            <div class="detail">
                <div class="box">
                    Div item 1
                </div>
                <div class="box">
                    Div item 2
                </div>
                <div class="box">
                    Div item 3
                </div>
                <div class="box">
                    Div item 4
                </div>
            </div>

        </div>
        <button id="add-new">
        Add
        </button>

JavaScript

$('.detail').each(function(i) {
  const targetNode = this;
  const countingDiv = $('.countDiv' + i);
  countingDiv.text(
    `There are ${
      $(targetNode).children('.box').length
    } divs inside parent box detail.`
  );
  createObserver(targetNode);
});

$('#add-new').click(function() {
	$('.detail').append('<div class="box">new new</div>')
})

function createObserver(targetNode) {
  const config = { childList: true };

  // Callback function to execute when mutations are observed
  const callback = function(mutationsList, observer) {
    for (var mutation of mutationsList) {
      if (mutation.type == 'childList') {
        countingDiv.text(
          `There are ${
            $(targetNode).children('.box').length
          } divs inside parent box detail.`
        );
      }
    }
  };

  // Create an observer instance linked to the callback function
  var observer = new MutationObserver(callback);

  // Start observing the target node for configured mutations
  observer.observe(targetNode, config);
}