push(@Children,$root->child_nodes); # could be push(@Children,$start_node_id); too, any starting point...
while(@Children)
{ $node = shift @Children;
  print $node,"->";
  my $Node = Node->new($node);
  my $next = $Node->first;
  while ($next)
  { print $next,",";
    my $Child = Node->new($next);
    push(@Children,$next);
    $next = $Child->next();
  }
 print "\n";
}