Maven aggregate unit test reports for multi-module projects

I have searched on this topic for a while and could not find an 'easy' way to combine the all unit tests files from multiple modules into one file html file. I have multiple (maven) projects which contain tests and all produce their own results on project level. I also have a parent project that I execute to invoke all the sub-projects. Ideally I would like to configure testng in the parent project to take all sub-project test files and combine them into one which will be stored on the parent project level.

If anybody has had a similar case I would love to hear how you solved this problem.

Thanks in advance.