If QProcess (or a separate class) would have an API to create process groups (job objects on Windows), then QProcess::kill() and tryTerminate() could be used to kill not only the started process, but also all child processes of that processes.

I.e. when terminating "make" then all the started compiler jobs should also be terminated.

Here's some more information about process groups:

http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC26
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/job_objects.asp