@DocChildren and @DocDescendants are possible if you just need a visual indicator in a view.
You could use an agent to spin through your docs. If a doc has a $REF field, it’s the UNID of the parent, which you could then use to get the parent and modify it.
You could put some script in a Response doc’s OnSubmit or PostSave event to edit the parent, which you could get with the doc’s ParentDocumentUNID property.
Just a few ideas off the top of my head…