if ( $expanded->[0]{$parent_key} ) {
  push @$expanded,  @$children;
}
else {
  # create cartesian product against existing $expanded
  $expanded = [
    map {
      my $row = $_;
      map {
        my $newrow = {%$row};             # clone expanded row
        @$newrow{keys %$_} = values %$_;  # add in parent_key details
        $newrow                           # return new row
      }  @$children;
    } @$expanded ];
}