You need to first fetch root level categories and then fetch their children in sub query.
Your query will list only terms which is not expected output.
First get parent terms and then fetch their children and process them separately.
Simply use built in wordpress function get_terms() by passing taxonomy as product_cat and parent as your desired product category id. This will return sub categories of given parent product category.
There are functions like get_the_term_list(), the_terms() to help you out formatting such terms.
For more than one depth you csn define functions and go recursively.