JSFiddle - React, Tailwind, and code Playground

HTML

<div id="explorer">
    <ul>
            <li id="phtml_1"> <a href="#" id="mechild_1">Root</a>
              <ul>
                <li id="phtml_2"> 
                    <a href="#" id="mechild_2">Root 1</a> 
                    <ul>
                      <li id="phtml_3"><a href="#" id="mechild_3">Child 1</a></li>
                      <li id="phtml_4"><a href="#" id="mechild_4">Child 2</a></li>
                      <li id="phtml_5"><a href="#" id="mechild_5">Child 3</a></li>
                    </ul>
                </li>
              </ul>
              <ul>
                <li id="phtml_6"> 
                    <a href="#" id="mechild_6">Root 2</a> 
                    <ul>
                      <li id="phtml_7"><a href="#" id="mechild_7">Child 4</a></li>
                      <li id="phtml_8"><a href="#" id="mechild_8">Child 5</a></li>
                      <li id="phtml_9"><a href="#" id="mechild_9">Child 6</a></li>
                    </ul>
                </li>
              </ul>
            </li>
          </ul>
  </div>

JavaScript

alert($('#explorer li[id^=phtml_]').length);