How to remove thumbnail sizes for specific post type on a child theme?

I think it might be possible, try adding this to your child theme’s functions.php

 add_action('init', 'remove_plugin_image_sizes');

function remove_plugin_image_sizes() {
    remove_image_size('image-name');
}

Here is the documentation: https://codex.wordpress.org/Function_Reference/remove_image_size