I'd probably just pass in the log file name, open it for exclusive append writing access with sysopen and flock, and sleep in the children waiting for an exclusive lock.

It depends on how many kids you have, what the system load is like, and how long the jobs take to process if this is a workable solution.

It seems conceptually easier to me than trying to coordinate lots of children trying to write to the same pipe in the parent, but that's just me.