Why is forEach not working for children?

Answer 1

Element.children is not an array. It is an object called an HTMLCollection. These do not have an array’s methods (though they do have the length property).

To loop through it, you'll have to convert it into an array, which you can do using Array.prototype.slice:

var children = Array.prototype.slice.call(document.getElementById("niceParent").children);

children.forEach(…);