adarkenigma 1] "live" user as suggested. install drivers
That is automated and not depending on user. When you start the Live session and select
non-free, the Live session and installed system will have the non-free NVidia drivers.
adarkenigma 2] also make new user (for my nephew)
Are you making the user? Why not setup the correct user from the start? Your nephew can always change the password of that user.
If not, then after you boot into the newly installed system, create a new user (systemsettings > account details > User Accounts), login as that user and remove the
live user from within that session.