Overwriting Parent Theme's Image Sizes

I’m using Raw by UDTHEMES as a parent theme for a site I am working on.

In the parent theme, there is the following image size:
add_image_size( 'udt-full-width-image', 870, 490, true );

And I am trying to overwrite it in my child theme like so (I have been told that I first need to unset it, and then set it again):

// enable post thumbnails
function sgr_filter_image_sizes( $sizes) {
	unset( $sizes['udt-full-width-image']);
	return $sizes;
}
add_filter('intermediate_image_sizes_advanced', 'sgr_filter_image_sizes');

function child_theme_image_size() {
    add_theme_support('post-thumbnails');
    add_image_size( 'udt-full-width-image', 870, 9999 );
    add_image_size('background-photo', 1680, 9999);
}
add_action( 'after_setup_theme', 'child_theme_image_size', 11 );

I’ve done this and then used the Regenerate Thumbnails plugin, but my images are still displaying at the wrong size.

Can anyone see what I’m doing wrong?