The parent entity (Document, in this case) should be extended with a one-to-one reference to your custom child entity. The one-to-one component includes an optional cascadeDelete attribute that will signal that the child should be deleted when the parent Document is removed.

Adding this one-to-one property is a legal data model change. It is a logical change only (similar to declaring an array) so it won't change the physical data model.

Here's a link to the documentation for reference (requires login).