An "Administrator" account has complete access to all other accounts through tools such as "sudo". To the extent that iCloud data is unsecured in the user accounts, it could be accessed by anyone with an administrator account. However, without the user's login password, access to information stored in the login keychain is not possible - so passwords to servers and websites and the like are reasonably "safe".
The simplest way to give reasonable secuity is to not make everyone an administrator - the standard setup requires administrator privledges to view files stored in other login account folders, so non-administrator accounts cannot view such files. If using non-administrator accounts is not feasable, one could create encrypted files and/or disk images to store private things.
The FileVault whole disk encryption does not do what you want, since it still allows full disk access to any administrator account.