my $count = scalar @$expanded;
$expanded = [ 
  map { 
    my $row = $_;
    map { 
      my $newrow = {%$row};
      @$newrow{keys %$_} = values %$_ unless $newrow->{$parent_key};
      $newrow }  @$expansion ;
  } @$expanded ];

push @$expanded,  @$expansion if $count == scalar @$expanded;


##</code><code>##

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 ];
}