Can’t override add_image_size() in child theme

Hello,
I tried to override several add_image_size()-instructions in my child-theme. Originally these function-calls are executed in hu_setup() in hueman/functions.php.

In my child theme I tried the following but it doesn’t work, it still uses the sizes provided by hueman:

function childTheme16to9Thumbnails() {
  	// Thumbnail sizes
	add_image_size( 'thumb-small', 160, 160, true );
    add_image_size( 'thumb-standard', 320, 320, true );
    add_image_size( 'thumb-medium', 520, 292, true );
    add_image_size( 'thumb-large', 720, 405, true );
    add_image_size( 'thumb-xlarge', 1000, 562, true );
    add_image_size( 'thumb-xxlarge', 1320, 625, true );
}
add_action( 'after_setup_theme', 'support16to9ThumbnailsMax1000px', 11);
# because of priority 11 this should be executed after hu_setup() but even priority 999999999999 doesn't work....