Hi @Gary_Bradshaw, welcome to the Asana Community Forum!
As it stands, it’s currently not possible to automatically create the subtasks with a due date connected to its parent task. I suggest you to upvote for this feature in this thread: Connect the due dates of subtasks to the due date of their parent task
Adding to what @IvanStaykov mentioned, you can find more information about Flowsana in this post: Flowsana: workflow automation for your Asana projects 
I hope this helps! Let us know if you have any other questions.