Cannot filter categories by parent category

To get direct descendants (i.e. children only) of the category, you need to make use of the parent parameter in get_categories()

The reason why you might not see your other category is that it does not have any posts assigned to it. To display empty categories, you need to set the hide_empty parameter to 0

Otherwise I do not see any problems with your code. You should also turn on debugging to check if you maybe have some kind of error if the above does not work