[Users] password protection

[Michael]

Imagine this situation: 

A family Linux computer where everybody is doing stuff, but the email program of the user account (owner) be protected by password (or some other way?) against misuse by nasty kids. The whole program needs to be restriced, with all folders, not only downloading mails, so just using the mailbox password is not an option.
Any ideas how to achieve this ?

(and the Mail folder needs to be hidden = dotfiled, i hopw this does not create problems for claws)