While it's possible to install Access Based Enumeration (which would hide individual folders/files within a share based on permissions), modifying permissions on your server at any level other than the share (which you change through the console) is likely to either A) not work for long, or B) cause problems on your server.
The recommended/supported method of dealing with this is to grant access to specific shares, and to separate content based on shares, so that a share your son has access to doesn't contain materials he shouldn't have access to.
I'm not on the WHS team, I just post a lot. :)
To clarify Ken's post: ABE only hides actual folders, it doesn't hide shares at all (which is what you're actually referring to, even though it's called Shared Folders on WHS). Plus installing it on WHS is not supported anyway. :)
