aimkeks

document.querySelectorAll('.parent').forEach((n, i) => {
  n.insertAdjacentHTML('beforeend', `<div class="child">${arr[i]}</div>`);
});

for (const [ i, n ] of document.querySelectorAll('.parent').entries()) {
  n.append(document.createElement('div'));
  n.lastChild.className = 'child';
  n.lastChild.innerText = arr[i];
}

arr.forEach(function(n, i) {
  const div = document.createElement('div');
  div.classList.add('child');
  div.textContent = n;
  this[i].appendChild(div);
}, document.getElementsByClassName('parent'));

const elems = document.getElementsByClassName('parent');
for (let i = 0; i < elems.length; i++) {
  const div = document.createElement('div');
  div.setAttribute('class', 'child');
  div.innerHTML = arr[i];
  elems.item(i).lastElementChild.insertAdjacentElement('afterend', div);
}

$('.parent').append(i => `<div class="child">${arr[i]}</div>`);