Per Marc Lehmann's feedback, I have documented how to avoid a

potential race condition when catching child processes with
sig_child().  Basically, if you want sig_child() to catch a process,
call the method in the same event handler that forked the process.
Otherwise POE::Kernel may reap the process before sig_child() is
called.

lib/POE/Kernel.pm

@@ -4525,14 +4525,19 @@ watcher can be cleared prematurely by calling sig_child() with just
 the PROCESS_ID.
 
 A session may register as many sig_child() handlers as necessary, but
-there may only be one per PROCESS_ID.
+a session may only have one per PROCESS_ID.
 
 sig_child() watchers are one-shot.  They automatically unregister
 themselves once the EVENT_NAME has been delivered.
 
 sig_child() watchers keep a session alive for as long as they are
 active.  This is unique among signal watchers.
 
+Programs that wish to reliably reap child processes should be sure to
+call sig_child() before returning from the event handler that forked
+the process.  Otherwise POE::Kernel may have an opportunity to call
+waitpid() before an appropriate event watcher has been registered.
+
 sig_chid() does not return a meaningful value.
 
   sub forked_parent {