basic show only sub categories of current category as facet options

<?php
// output only sub categories of current woocommerce product category
add_filter( 'facetwp_facet_render_args', function( $args ) {

    if ( 'my_facet_name' == $args['facet']['name'] && is_product_category() ) {  // replace 'my_facet_name' with the name of your facet

        $current_term = get_queried_object_id();

        foreach ( $args['values'] as $key => $row ) {
            /* to also include current term in chocies add this to if:
               && $row['term_id'] != $current_term
            */
            if ( $row['parent_id'] != $current_term ) {
                unset( $args['values'][$key] );
            }
        }

        $args['values'] = array_values( $args['values'] ); // fixes indexes
    }

    return $args;
});