Proper setup for automatic updates for my kid's computer
Solution 1:
A non-admin user (without sudo access) is not supposed to install packages or perform updates (that is the whole point of having non-admin users in the first place, so that many people can use a computer, but they won't be able to break the installation even if they unknowingly try to do something wrong).
You can occasionally log in from your (admin) account to perform the updates. Alternatively, you can set up unattended upgrades in the admin account, so that Ubuntu would be automatically upgraded when you use the computer.