Terminate a process tree (C for Windows)

This has been asked before but I can't find a definitive answer, in code.

I open a process, ProcessA (with PID 1234). This process opens a child process, ProcessAB (PID 5678). After I'm done I terminate ProcessA but I still have the lingering of ProcessAB.

How do I terminate the whole process tree? What I mean, how do I make sure that if I terminate the process I opened I am also terminating all the associated processes?

Thanks

Code is appreciated.