You should make use of the tablet's child restrictions features to help your child control himself until he demonstrates the maturity to have more permissions. 

I would suggest to make yourself the administrator of the tablet. The child should not have permissions to install apps, and only be able to use limited apps. This way, you can control how much time they use the device, and what apps are installed. He should have to earn the privilege to use the tablet.

Which tablet do you have? I can suggest some concrete ideas.