This question is a derivative of a previous question: How to version resources that are shared across projects
I have a project that contains code that is consumed
Go to one specific file you know has changed in your "parent" project. Try merging just that file. Don't check anything in; just see what happens.
Something to watch for: The merge tools will not include files that have been added after you branch. You have to branch new files explicitly before you can merge any further changes. If a file is added to both parent and child folders without using a branch operation, the merge tools don't treat them as versions of the same file (and you can't merge changes between them).