If you kill the cmd window, the child process nbproxy will also die.
There might be exceptions, but this is what I expect.
The next question probably is "What will happen when the proxy dies?" It all depends on which proxy you killed. If you killed the nbproxy that is started by nbjm you would end up with nbjm in a bad state since nbjm is depending on inter-process communication facilitated by the proxy. If you kill the cmd processes, you will kill the nbproxy process which could mean that you have essentially hung up on some key communication in NetBackup. Your best course of action would be to restart NetBackup.
Is there a reason why you want to kill these cmd processes?