Shift hierarchical facet indexing to skip top level parents

<?php
/** changes depth and parent_ids
 ** to allow a hierarchy to start with
 ** first level children instead of top level parents
 **/

add_filter( 'facetwp_index_row', function( $params ) {
    if ( 'my_facet' == $params['facet_name'] ) { // 'my_facet' should be replaced with your facet name
        if ( 1 > $params[ 'parent_id' ] ) { // this will be top level parents
            $params[ 'facet_value' ] = ''; // don't index
        } elseif ( 2 > $params[ 'depth' ] ) { // first level children need depth and parent id changed
            $params[ 'parent_id' ] = 0;
            $params[ 'depth' ] = 0;
        } else { // other children just need depth adusted
            $params[ 'depth' ] = $params[ 'depth' ] - 1;
        }
    }
    return $params;
});