AngularJS Directives why don't any class or style changes to children of the element apply?

I'm currently using a directive for DOM changes for my application. I would like a button in element to hide on a state change.

HTML:

<div account layout="row" layout align="start center">
    <md-button id="accountIcon" class="md-icon-button">
        <md-icon aria-label="account" layout-align="center center">
            <i class="material-icons">account_circle</i>
        </md-icon>
    </md-button>

Directive:

angular.module("app")
.directive('account', ($document) => {
    return {
        restrict: 'A',
        link: (scope, element, attrs) => {
            scope.$on('hideAccount', () => {
                angular.element($document[0].querySelector('#accountIcon')).addClass('ng-hide);   
            });
        }
    };
});

The event is received and function is fired. The following also don't work.

angular.element(document.querySelector('#accountIcon')).addClass('ng-hide'); 
angular.element(document.querySelector('#accountIcon')).css('display', 'none'); 
angular.element(document.querySelector('#accountIcon')).addClass('hide'); 
var accountIcon = element.children()[0];
accountIcon.addClass('ng-hide');

where:

.hide {
    display: none;
}

Hopefully you get the gist. The following did work:

element.addClass('ng-hide')

Although obviously, I'm not looking to hide the entire element because there will be more buttons added soon. (not just an account icon). Any help or documentation appreciated. Thank you.

Did you check the console for errors?

element.children()[0] is a DOM Node, which doesn't have an .addClass() method. To use the .addClass() method, just do this:

element.children().eq(0).addClass('ng-hide');

But the question remains - why are you going out of your way to use DOM manipulations when you don't have to? Couldn't you just use the ng-hide directive within your directive? Accessing elements by ID and doing all these DOM manipulations seems unnecessary.