Unless I'm missing something, it sounds like the child process continues to run after system() returns (e.g. you are using '&' in your system call or the child itself spawns other children that live past it). The reason I say this is because NFS is no different than other filesystems -- the write calls to it must complete before they return to writing process. So this isn't really an NFS issue and may eventually bite you even without NFS.

If you have control of the child's code, you should be able to make sure it finishes completly before returning from system by making sure it isn't doing anything in the background.

If you don't control the child's code, but know all of the process PIDs, you can wait for them to die off. Otherwise, you'll probably have to pick a method of polling (as the other monks here have mentioned).