<schema xmlns="http://www.ascc.net/xml/schematron" >

     <pattern name="a child of selected elements">

          <rule context="CCC">

               <assert test="parent::*[name()='BBB' or name()='DDD']">Element CCC must not be a child of element

                    <name path="parent::*"/>

               </assert>

          </rule>

     </pattern>

</schema>