Chris,
I have discovered that seems to be an order of operations requirement here (SourceItem->childItem no related method). To make this work, you have to create the source item and the relationship, which then drives the creation of the child item type.
I am yet to find a way to do this by modifying currently existing item types. I confirmed this behavior by attempting to duplicate the properties of the stock models/products structure into my existing structure. This did not succeed. I was able to follow the process described above to success.
Is there something I'm missing to make this work with pre-existing item types, or is it not feasible?
