So I was finally able to find what I needed:
$loop2 = new WP_Query([
'post_type' => 'service',
'numberposts' => -1,
'meta_key' => '_wpcf_belongs_service-category_id',
'toolset_relationships' => [
'role' => 'child',
'related_to' => $parent_id,
'relationship' => ['service-category', 'service']
]
]);
https://wp-types.com/documentation/customizing-sites-using-php/displaying-child-posts/