In unoxml when a uno xml element is created, e.g. 

xDoc->getDocumentElement() in 
desktop/source/deployment/registry/package/dp_description.cxx

Each such UNO xml node is implemented with a CNode
every CNode takes a uno::Reference to its owner document for the nodes lifecycle
i.e. m_rDocument = getOwnerDocument() in unoxml/source/dom/node.cxx

but each CNode also calls "addnode" on its owner to add a uno::Reference to that
node from the ownerdocument for its lifecycle

Each node holds a reference to the ownerdocument, and the ownerdocument holds a
reference to each node which means that the whole cycle doesn't get released.

In this case I think it should be sufficient for each child node to hold a
reference to the parent document.