单击按钮时不会触发Javascript功能

Question

以下是代码：

<script>
function myFunction() {
var x = document.getElementById("myDIV");
var y = x.getElementsByClassName("child");
var i;
for (i = 0; i < y.length; i++) {
    y[i].style.backgroundColor = "red";
    }
}
</script>
<style>
div {
    border: 1px solid black;
    margin: 5px;
}
</style>

<div id="myDIV">
  <p class="child">A p element with class="child" in a div.</p>
  <p class="child">Another p element with class="child" in a div.</p>
  <p>A p element in div with a <span class="child">span element with class="child"</span> inside of it.</p>
</div>

<p>Click the button to add a background color to all elements with class="child" inside the div element with id="myDIV".</p>

<button onclick="myFunction()">Try it</button>

https://jsfiddle.net/jmgrant15/3dcof7ox/14/

＆＃34;孩子＆＃34;类按钮应该在按钮单击时变为红色。有什么想法，为什么这不起作用？

谢谢！