我想交换两个孩子div的位置。

<div class="container">
  <div class="child">Child 1</div>
  <div class="child">Child 2</div>
</div>

.container {
  display: flex;
}
.child {
  width: 50%;
  height: 100px;
  border: 1px solid black;
}
.child:nth-child(1) {
  order: 2;
}
.child:nth-child(2) {
  order: 1;
}

<div id="container">
  <div id="child1">Child 1</div>
  <div id="child2">Child 2</div>
</div>

var container = document.getElementById("container");
var child1 = document.getElementById("child1");
var child2 = document.getElementById("child2");

container.insertBefore(child2, child1);