get element by second child ID - element

Question

I want to change the second ID "myDIV" second p tag(index 4) class "child" backgropund color. But my script does not work. How to get the second child ID?
<div id="myDIV">
<p class="child">First p element with class="child" in a div (index 0).</p>
<p class="child">Second p element with class="child" in a div (index 1).</p>
<p class="child">Third p element with class="child" in a div (index 2).</p>
</div>
<div id="myDIV">
<p class="child">First p element with class="child" in a div (index 3).</p>
<p class="child">Second p element with class="child" in a div (index 4).</p>
<p class="child">Third p element with class="child" in a div (index 5).</p>
</div>
<button onclick="myFunction()">Try it</button>
<script>
function myFunction() {
var x = document.getElementById("myDIV")[1];
x.getElementsByClassName("child")[2].style.backgroundColor = "red";
}
</script>

Answer 1

Element IDs should be unique within the entire document.
You can replace id="myDIV" with class="myDiv"
Then change your script as
<script>
function myFunction() {
var x = document.getElementsByClassName("myDIV")[1];
x.getElementsByClassName("child")[1].style.backgroundColor = "red";
}
</script>
<div class="myDIV">
<p class="child">First p element with class="child" in a div (index 0).</p>
<p class="child">Second p element with class="child" in a div (index 1).</p>
<p class="child">Third p element with class="child" in a div (index 2).</p>
</div>
<div class="myDIV">
<p class="child">First p element with class="child" in a div (index 3).</p>
<p class="child">Second p element with class="child" in a div (index 4).</p>
<p class="child">Third p element with class="child" in a div (index 5).</p>
</div>
<button onclick="myFunction()">Try it</button>
<script>
function myFunction() {
var x = document.getElementsByClassName("myDIV")[1];
x.getElementsByClassName("child")[1].style.backgroundColor = "red";
}
</script>