You can get multiple child of a parent element by using JavaScript Array forEach() or you can use for loops. Also we can sum of all the children elements width. Here is the example:

//html structure
<div class="parent">
   <div class="child">Child 1</div>
   <div class="child">Child 2</div>
   <div class="child">Child 3</div>
</div>
<div class="parent">
   <div class="child">Child 1</div>   
   <div class="child">Child 2</div>   
   <div class="child">Child 3</div>
</div>


//JavaScript code

<script>
const parentClass = document.querySelectorAll(".parent");
const childClass = document.querySelectorAll(".child");

 parentClass.forEach(item => {
   x = item.children;

   //Sum of children width value
   let sum = 0;
   for (let i = 0; i < x.length; i++) {
          sum += x[i].offsetWidth;
          getChildren = x[i];
    }
    // find all the children elements
    console.log(getChildren);

    // Find all child elements total
    console.log('Total width of child elements :' + sum);
  }

</script>