Get immediate first child element
Both these will give you the first child node:
console.log(parentElement.firstChild); // or
console.log(parentElement.childNodes[0]);
If you need the first child that is an element node then use:
console.log(parentElement.children[0]);
Edit
Ah, I see your problem now; parentElement is an array.
If you know that getElementsByClassName will only return one result, which it seems you do, you should use [0] to dearray (yes, I made that word up) the element:
var parentElement = document.getElementsByClassName("uniqueClassName")[0];