My son's user account and my user account on my iMac are linked/synced through iCloud. How do I unlink them? So, on my home iMac - I have separate user accounts for me, my wife, and my two kids (ages 5 &amp; 7).

This has worked great - my young children only have access to age-appropriate sites, and bookmarks for all their favorites.

However - after I upgraded my iMac to Yosemite - I noticed that, when I first logged into my child's account to confirm that everything worked - I was prompted to log into iCloud using my Apple ID. 

I didn't really understand this - I was logged into iCloud on MY user account; why should I have to log in again for my son's user account?

Nevertheless - and foolishly - I did log in again.

Now - my son's user account and my user account are linked and synced through iCloud. He can see all my bookmarks, and I can see all of his. I'm sure everything else that's stored on iCloud drive are also synced between our two user accounts.

So - I guess what I need to do is "unlink" my iCloud account from my son's user account. However - I can't figure out how to do that.

Any advice or suggestions?
On your sons account you should be able to open up System Preferences, choose iCloud (you should see your account logged in here). From there just click sign out. It will ask if you want to keep synced bookmarks, contacts, notes, etc you can then choose to delete them or keep them on that account.

Edit: Short answer: When you remove iCloud Drive from your sons account, your files will still remain on your account.
Longer answer: I tested out the iCloud Drive problem by creating a new user account managed by parental controls on my Mac. After I created this account, it asked me to sign into my Apple ID, which I did. I then went to sign myself out of the managed user account. I had the same thing occur, it said that my iCloud Drive documents will be deleted from this computer. I clicked delete, and removed them, as well as contacts, bookmarks, etc. When I logged back into my main (administrator) account all of my iCloud Drive files were still there, it is safe to delete iCloud Drive from that account.