Matthias, do you want to display tasks that have subtasks that are completed, or, possibly that have all subtasks completed?
Something like: Parent task not completed and at least one subtask completed.
Or, parent task not completed and all subtasks completed.
Or, it doesn't matter whether the parent task is completed or not, as long as there are some (or all) subtasks completed.
If this is the case, then I don't know how to do it in Toodledo. In a search for Tasks with subtasks, you can't specify whether the subtask is completed or not.