How to test if a specific page is the front page?

You could try to replace:

if (is_front_page()) {
    $menuIdOfCurrentPage = 0;

with a check for the page_on_front option:

if( 
       is_int( $postID ) 
    && $postID > 0 
    && $postID === (int) get_option( 'page_on_front' ) 
) {
    $menuIdOfCurrentPage = 0;

Here we added a check to see if $postID is a positive integer, because the page_on_front option is 0 if no page is selected as a frontpage.

Then consider renaming $menuIdOfCurrentPage to $menuIdOfPage.

In PHP 7, we could for example use strict scalar type declerations:

function getMenuChildren( int $menuId, int $postId )
{

where it would throw a TypeError when used with non integer input arguments:

Fatal error: Uncaught TypeError: Argument 1 passed to
getMenuChildren() must be of the type integer, none given, called in
[…][…] on line 10 and defined in […][…]

using declare( strict_types = 1 );.