Thank you for the suggestion. :-)
Right now LSWS does graceful restart, means that the old LSWS instance still need the old PHP processes before it finishs all pending requests. Let new LSWS instance to manage old PHP processes may break the graceful restart.
I am thinking to have the watch dog process of old LSWS instance to manage old PHP processes as it knows when the old LSWS process exits, even crashes. Only need to find the best way to inform watch dog process about PIDs of children processes started by LSWS worker process.