I am sorry that you have lost your subtasks, but this is the intended behavior.
From: http://www.toodledo.com/info/subtasks.php
When you complete a task that contains uncompleted subtasks, Toodledo assumes that those uncompleted subtasks are now irrelevant. We will no longer display nor remind you about them. If these tasks are important, you should move them out of their parent task before you mark it as completed.