Using signal in kill method

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I want to write a system call for FreeBSD kernel which get a PID and do depth-first-search on its child then return all of the PIDs.

I'v read about load kernel module and I understand how to right them.(ref to this link) and I understand that kill method is the useful system call to send the signal for a children process. (ref to this link) but I don't know which signal can I use, because all of the signals are terminate or stop the process but I want only traverse them.
 
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