How to prevent chid from installing apps from unknown sources?

[Android Central Question]

Hey guys,

I bought a ZTE Blade A6 recently for my 9-years old brother but I noticed that he downloaded games that are 16+/18+ multiple times even though I have told him more than enough times not to do it anymore.
Then I activated the parental controls for the google play store.
The problem is: Every time our 12-years old cousin meets him, he installs TutuApp/Aptoide with APK's from the internet and downloads those games again.
I already tried to talk to his cousin but he won't stop.
I'm pretty sure that even if I block the installation of apps from unknown sources in the android settings, he'll just figure it out and remove the block again.
Is there a way from actually preventing him from accessing these settings?
I know that there are apps like AppLock but they can just be uninstalled pretty easily.
If there's no option, and I notice that he does it again, I'll take his phone away and give him a cheap non-smartphone instead. I'd honestly prefer to avoid that though.

 

[B. Diddy]

Welcome to Android Central! I would strongly suggest setting up Family Link. If you previously set up an account for your brother that doesn't comply with Google's policies (i.e., you're not supposed to be able to have your own standard Google account unless you're 13 or older), you'd have to set up a new Family Link account for him. It allows greater control over his account, and locks the phone down more tightly. Learn more here: https://families.google.com/familylink/