Technically the simplest thing is to disable internet on the tablet. You can do this easily: wireless networks normally require the password, GSM networks require SIM card that can be removed.
This is reliable, even professional software engineer cannot connect the protected wireless network without knowing the password.
Before doing this, I would suggest to install selected educational educational apps. They are not addictive enough for this to be a problem and this way should be much more useful than just cutting the tablet into two.