Hi Rasmus,
What is the relationship between ParentEntity and ChildEntity?
Based on your description, I assume it would be Parent-1_0..1-Child, but in that case it would not make sense to call Delete and then create another Child entity and add to Parent because the Deleted entity still exists and the new Child entity would have the same PK (on a one to one relationship, the FK is the PK).
As a matter of fact, I'd get an exception for duplicate keys when trying to associate the newChild to Parent.
Please provide enough detail so we can try to reproduce the problem.