Modify Child Object During Gridview OnUpdate Using ObjectDataSource?

Backgroud: I'm using NHibernate for my model layer, and I have a HTTP module taking care of instantiating the session at the beginning of each request and cleaning it up at the end of each request (i.e. Session Per Request). In this case, I have two object types:

ParentItem - an object with a bunch of properties and a collection of ChildItems. ChildItem - an object with properties including a DateTime (EffectiveDate) and an unenforced FK pointing to a completely different database. The ChildItem class contains a reference back to the parent as well (many-to-one) While the ParentItem has multiple ChildItems in its collection, I'm only generally interested in the latest ChildItem in the collection.

Desire: Want a databound control (GridView or ListView, I don't care which) that allows me to Add/Edit/Delete ParentItems from my datasource. I also want to have the ability to set a new "latest" ChildItem as part of the edit/update.

Issue: I can't seem to access the underlying DataItem from the GridView/ListView in the OnItemUpdating handler (which isn't unexpected, since the data is now in the viewstate). What I thought I could do would be to load a ParentItem from my session using the ID from the databound control, create a new ChildItem, add it to the ParentItem, and then save the ParentItem. Since NHibernate caches the data, the load should give me a copy from cache (no roundtrip to the DB) and I'd either be doing this before the ParentItem was saved back (thus no changes committed to ParentItem, just to ChildItem) or after (thus the cached version is still the same, and my new object will match the updated version). What I get when I do this (for ParentItemID=1):

a different object with the same identifier value was already associated with the session: 1, of entity:

NameSpace.ParentItem

on the line:

Session.SaveOrUpdate(parentItemInstance);