I have the method (on the ListView)  set as FillByparentPrimaryKey(System.Int32) and PopulateOnFormLoad set to manual.

I reckon I need to call the FillByParentPrimaryKey() somewhere to refresh the children and am wondering where is the best place to put this code. I could be refreshing ,either using the navigation buttons, Adding a new Parent,Deleting a parent or  doing a search etc.

Also, is just calling FillByParentPrimaryKey() sufficient or do I also need to call any Refresh() code

I can understand why you would use a FillByParentPrimaryKey(Int) but when would you use GUID ?? I am unclear as to what a GUID is, other than this would be a term used for a surrogate type key . 

4d9895f1-7d0d-4003-9a28-389624b36c58

This is used in instances when you may need to use replication as it makes the synchronization of primary keys far easier.  If you can avoid using GUIDs for PKs you are better off as there is more overhead.  But this one general purpose of using a GUID.  There are other uses as well, but for the intent of this converstaion that would be one of the few uses.