tungens

const parent = document.querySelector('.parent');
const toWrapClass = 'child';
const wrapperTag = 'div';
const wrapperClass = 'wrapper';

[...parent.children].reduce((wrapper, n) => {
  if (n.classList.contains(toWrapClass)) {
    if (!wrapper) {
      wrapper = document.createElement(wrapperTag);
      wrapper.classList.add(wrapperClass);
      n.before(wrapper);
    }
    wrapper.appendChild(n);
    return wrapper;
  }
  return null;
}, null);

Array.prototype.reduce.call(
  parent.querySelectorAll(`:scope > .${toWrapClass}`),
  (acc, n, i, a) => (
    n.previousElementSibling !== a[i - 1] && acc.push([]),
    acc.at(-1).push(n),
    acc
  ),
  []
).forEach(n => {
  const wrapper = document.createElement(wrapperTag);
  wrapper.className = wrapperClass;
  parent.insertBefore(wrapper, n[0]);
  wrapper.append(...n);
});

const toWrapSelector = `.${toWrapClass}`;
const wrapperHTML = `<${wrapperTag} class="${wrapperClass}"></${wrapperTag}>`;
for (
  let curr = parent.firstElementChild, next = null, prev = null, wrapper = null;
  next = curr?.nextElementSibling, curr;
  prev = curr, curr = next
) {
  if (!curr.matches(toWrapSelector)) {
    continue;
  }
  if (!prev?.matches(toWrapSelector)) {
    curr.insertAdjacentHTML('beforebegin', wrapperHTML);
    wrapper = curr.previousSibling;
  }
  wrapper.insertAdjacentElement('beforeend', curr);
}