[selowitch]

My wife, my son, and I share a Mac for which I am the Admin and they are mere mortal (Standard) users. But I still can't access files in their user directories and can't change permissions on those folders through the GUI (I imagine I could probably do it via the CLI, but that's a drag if it's possible to do it some other way). What am I missing here? Maybe I need to log in as the "root" or superuser?

[Big Mac]

Regular administrators are only supposed to have enough power to administer the Mac. They are not granted power over all the files on the system. I have never needed to login as root. I have used sudo, though, and I have also logged into my admin account through the Terminal of another user. If you explain what you're trying to accomplish, people will tell you how to do it.

[barney ntd]

If you do a "Get Info" on their folders, show the "More details" under Permissions, then click the padlock, you can do what you like. Normally, you should change the group, not the owner, though you may need to change the owner temporarily in order to set things up as you like.

[Big Mac]

And you can temporarily make another user an admin so that that user can change ownership through the Finder.