Hi Simon!
Thanks for getting in touch with us. Let me try to help you with this topic.
First, please let me note that we are fairly limited in how much support we can give for custom development questions like that.
That said, we always like helping out and at least point users into the right direction as much possible.
The tribe_get_event_categories() function doesn’t allow a parent argument to be passed in order to only fetch specific child categories. You can try using the wp_get_post_terms() WordPress function that allows you to set a parent argument > https://developer.wordpress.org/reference/functions/wp_get_post_terms/
I hope that helps. Let me know if you have any follow up questions.
Best,
Victor