document.querySelectorAll('.parent').forEach((n, i) => {
n.insertAdjacentHTML('beforeend', `<div class="child">${arr[i]}</div>`);
});
или
for (const [ i, n ] of document.querySelectorAll('.parent').entries()) {
n.append(document.createElement('div'));
n.lastChild.className = 'child';
n.lastChild.innerText = arr[i];
}
или
arr.forEach(function(n, i) {
const div = document.createElement('div');
div.classList.add('child');
div.textContent = n;
this[i].appendChild(div);
}, document.getElementsByClassName('parent'));
или
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);
}
или
$('.parent').append(i => `<div class="child">${arr[i]}</div>`);