First are you using the SimplePrincipal from Jboss or your own Principal Object ?
What is the role your are associating with your Principal object when you log in as Child ? My guess it it should be C ( as per your web.xml) .
If you are rolling out your ow authentication, i guess you would be subclassing the AbstractServerLoginModule. If Using your own Principal, subclass SimplePrincipal. In the AbstractServerLoginModule there is a method addRoles() i believe. Override it with your roles.
Maybe you can post the jaas login module code ...