How do I keep the user on a specific form for the new child record?

Have you tried the workflow of adding the grandchild instead of saving & closing when entering the child?  As long as your key field is not a number, this should work just fine.  Then you do would not need to worry about the redirect to the original parent from the child and trying to stay on the same dfid10.

Make sure EVERY table in your App has the [Save parent record automatically when a child record is createduto save when adding a child] box in the Advanced Table Settings turned on.