There is currently no way to have the subtasks completed when a task is marked complete. So I built a tool to, at least, be able to do this afterward, as a cleanup mechanism.
The rules are:
- the parent task is completed
- the subtask is unassigned
- the subtask is inside project(s) the user has chosen
- going only one level down
I hope that will be useful! https://minimalist-work.com/asana/subtask-cleaner