I need this solution, but I'm a bit confused with the instructions:
http://crmtipoftheday.com/2014/06/25/create-a-11-relationship/
It says to create a 1:N and N:1 relationship between the two entities. Doesn't this already happen by default? And it says to create a lookup on both "entities." The child entity already has the lookup by default when you create this. SO, I used that one and created one on the parent back to the child.
The workflow to populate the lookup from the parent to the child is my problem. I assume you should be populating the parent with the PK from the child, but I can't see it in the workflow on create of the child.