aimkeks

const containers = document.querySelectorAll('.parent');
const tag = 'div';
const className = 'child';

containers.forEach((n, i) => {
  n.insertAdjacentHTML(
    'beforeend',
    `<${tag} class="${className}">${arr[i]}</${tag}>`
  );
});

// или

for (const [ i, n ] of containers.entries()) {
  n.append(Object.assign(document.createElement(tag), {
    className,
    innerText: arr[i],
  }));
}

// или

for (let i = 0; i < containers.length; i++) {
  const el = document.createElement(tag);
  el.classList.add(className);
  el.textContent = arr[i];
  containers[i].insertAdjacentElement('beforeend', el);
}

// или

(function add(i, n = containers.item(i)) {
  if (n) {
    n.appendChild(document.createElement(tag));
    n.lastChild.setAttribute('class', className);
    n.lastChild.insertBefore(new Text(arr[i]), null);
    add(-~i);
  }
})(0);