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

Just remove  & Rurl() from the end of the formula

But that loads the default form for the child record. I want a specific form loaded.

Or more accurately stated... I want it to stay on the previously loaded specific form of the child record, rather than load the default form for the 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.

Laura is correct, but the real question here is if you can avoid have multiple forms and instead have forms rules and just one form.