having this project structure:
pom.xml[type=pom, defines the pmd plugin in the reporting section]
running "mvn site" on the parent project generates all reports on the parent target/site/ folder (Project License, CheckStyle, FindBugs, ...), except PMD and CPD, which get generated in the two modules target/site/ dirs, and thus do not show up in the main report.
tested with maven PMD plugin versions 2.6 and 2.7.
if more info is needed, or testing, tell me please.
i attached a a zip file with a quite minimal test project.
The zip contains a folder TestProject with the above structure inside. try running "mvn site" in folder TestProject, and have a loot at: