I create a transient parent and then add a transient child to a persistant collection in the parent. I then call SaveOrUpdate on the parent and then make a change to the child object in the collection. The changes to the child aren't picked up once I do a flush...why?

Thanks in advance.

Make sure that you enable cascading...
Read:
7.7.2. Persistence by Reachability
8.3. Cascades

I believe the cascades are enabled correctly. And if I add children to the collection after I save the parent everything works fine...it's just the children that are added to the collection after the save but before the flush that are the issue.