Mike if you use a work item attribute of type Item to select and store other work items, then there are no references/relationships between the work items involved. The work item basically stores a reference to the other work item in the attribute value (list). It does not create any new links/references such as parent/child or whatsoever on any of the involved work items.

If you want to have parent/child, you have to use the parent/child linking. If you want to use the work item attribute type to maintain parent/child, you would have to create a follow up action, that creates the links if they are not available yet. Please be aware, that you would have to handle exceptional cases your self in this case.