How to track multi-process program?

[Icegrave0391]

Hi, I tried to attach the program by using the following command:

pin -follow-exec -t libdft.so -- /path/to/my/exec

Specifically, my traced program is a nginx web server and I'd like to track the taint propagation in its worker processes.

Currently, my naive approach is to use LOGD defined in libdft to track the syscalls and taint tags.
However, it seems the instrumentation stops when the master process spawns and executes its work process, even though I've set the option -follow-exec.