<div class="container">
  
  <div class="parent">
    parent 1
    <div class="child">
      child 1
    </div>
  </div>

  <div class="parent">
    parent 2
    <div class="child">
      child 2
    </div>
  </div>
  
</div>

.container {
  padding: 1rem;
  background: antiquewhite;
}

.parent {
  position: relative;
  background: burlywood;
}

.child {
  position: absolute;
  padding: 1rem;
  background: chocolate;
}

const parrent = document.querySelectorAll('.parent')
parrent.forEach(el => {
  el.style.height = `${el.querySelector('.child').offsetHeight}px`
})