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 doSince you have? an Android tablet, I would recommend:
Factory reset the device to start with a clean slate.
Create yourself as the Owner and don't share the passcode with your child.
Create another user for your child as a Restricted Profile. This allows you to control what apps the Restricted User can use. Don't give permission for the Google Play store, and the Restricted User won't be able to install apps themselves. Instead, you install the app in the Owner account and give permission to the Restricted account.
You can suggest some concrete ideasfollow the directions at http://www.laptopmag.com/articles/parental-controls-android .
Unfortunately, Android doesn't have a built-in time restriction feature. You can try a third party tool such as https://play.google.com/store/apps/details?id=com.screentime.rc , but those usually have monthly subscription fees.
Even for a computer, you should create a separate non-administrator account for others in your household so they can't install programs without your permission. Someone could unknowingly install malware in a blink of an eye.