FacetWP – Only direct children

/**
 * Only include direct children in category facets
 *
 */
add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'tax/category' == $params['facet_source'] ) {
        if ( 0  !== $params['depth'] ) {
            $params['facet_value'] = ''; // don't index this row
        }
    }
    return $params;
}, 10, 2 );