For pre-Android 4.3, if a tablet (or any device) has root, will all its user accounts have root access? This question is for Android 4.2 and below.

If a tablet (or any applicable Android device) has root, will all user accounts have root access? Will the SuperUser app also be installed for all users?

Also, would it be possible to disable root access for one of those users?

I no longer have a tablet, so I can't confirm this behavior myself.
With rooted device, only one account called root with both user and group id of zero is present, does not matter how many accounts there are. Similar to Mac OSX, Linux counterparts. 

There is a singular Superuser.apk, all accounts can run an app that requires root privileges. It boils down to what apps have the permission granted. AFAIK Superuser.apk is not multi account aware? if I am wrong feel free to correct me

Multiple accounts are only available on tablets, however cannot confirm either as do not have one. :)