Tool to query dtd/schema for valid element children|attributes

I'm looking for a tool that will enable me to query the docbook dtd -- 
given a particular element, I want to get lists of its valid children 
and attributes.  Ideally it could also provide information about an 
attribute's content type.

So far I've tried the perl modules XML::DTDParser, XML::ParseDTD and 
XML::SMART::DTD.  All of them work with other DTDs, but fail to process 
http://oasis-open.org/docbook/xml/4.2/docbookx.dtd (or the local copy on 
my system).

Can anyone point me to a tool that might suit my needs?  If there's a 
tool that can query RNG, I can translate the DTD using trang.

Regards,
David.