The great thing about Ubuntu ( and Linux in general ) is that users
are limited in what they can do to mess up as they will need to know
the system password to do anything that's likely to 'mess up', but
this doesn't get in the way of just *using* the PC. So, create a user
account for them if you wish but don't let them know your system
password ( i.e. the one you created when installing Ubuntu ).