Task Scheduler only killing cmd.exe but not its child processes
Try using "Taskkill /T" on the command line. (/T = “…Terminates the specified process and any child processes which were started by it…”)
If you have multiple processes running (we often have 5 or more "powershell.exe" running) then add the "Command Line" column to the "Details" tab in Task Manager. That should make it clear which Process ID is the one you want to kill off
Processes are independant in Windows - Killing a parent process will not automatically terminate a child process.