I'm not clear on what you mean by "get the attributes for child MBO from parent MBO", can you elaborate on what your expectation is?
There are, in general, two scenarios here, inbound and outbound. For outbound, you are supplying parent and child (and grandchild) MBOs as parameter values to an operation or object query. For inbound, you are receiving MBO instance data as a response from an object query. Which scenario is it you are having difficulty with? For outbound, there should be all kinds of content in that file in terms of wrapper objects to make using the different MBO types easier to use. For inbound, you'll be implementing processDataMessage yourself, either by taking the incoming message string and converting it into a WorkflowMessage or by parsing it yourself in whichever manner is most appropriate for your solution.