I believe (and this is based on hazy recollection from glancing at some article about the guts under either FAM or possibly BSD's kqueue comparing it against FAM) that the notifications are tracked at the inode level in the kernel. Each tracked inode has to have a pointer set to the notification routine, and if it's not NULL it gets called. The kernel doesn't have any way to know that you're even interested in children of the inode (hypothetically it could walk up the parent chain, but that'd add even more overhead to file access). I think you're pretty much going to have to explicitly walk the directory tree and request notification for all child directories.