I'll be on vacation for a short while starting tomorrow.
and since my parent's laptop died everyone will be using mine.

since my dad and sister never worked with linux before I though it be more save to make an account to be used during that week, which is not a sudouser.
so they can't kill my OS (so to speak.... )

problem is, that account can use sudo and kdesu to get root privileges, and I don't want it to.
but it's not part of the admin or the sudo group, a.k.a. it shouldn't be able to use them.

You will probably have to edit the sudoers file. Please be aware that there are some very specific procedures for doing that. I'm no expert, since I have a single user machine, and no one else is interested in using it, but I suspect that may be the problem.