Display all subcategories from parent category

get_categories() uses get_terms() so the array argument you can pass it will be the same.
From the documentation the array can have a property hide_empty, by default its true.
I’m guessing that those categories have no posts attached to it, if that is the case you will need to set it to false.
The code would be like this.

get_categories([
    'child_of'   => 505,
    'hide_empty' => false
]);

Update

If you want to order the results you can add order to the array, the default value is ASC so you will need to set it to DESC.

get_categories([
    'child_of'   => 505,
    'hide_empty' => false,
    'order'      => 'DESC'
]);

You can check WP_Term_Query::__construct( string|array $query = ” ) for all the available arguments you can use.