JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="log">

</ul>
<div id="parent">

</div>
<div id="newParent">

</div>

CSS

#parent {
  background: black;
  color: white
}

JavaScript

function setChildren(nr) {
  for (let i = 0; i < nr; i++) {
    let child = document.createElement('p')
    child.innerText = 'child Node'
    document.getElementById('parent').appendChild(child)
  }
}

function addLogMessage(msg) {
  let message = document.createElement('li')
  message.innerText = msg
  document.getElementById('log').appendChild(message)
}

setChildren(10)

let parent = document.getElementById('parent')
let newParent = document.getElementById('newParent')

parent.childNodes.forEach(child => newParent.appendChild(child))

addLogMessage(parent.childElementCount)
addLogMessage(newParent.childElementCount)