Hi Thomas,
It should not be too difficult I think. I have quickly made a custom API code that changes the search parameters before the search, and if finds parent terms excluded, it excludes child trems as well.
Add this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
Let me know if you need help with this.