Need to set up a phone with limited permissions to help with a learning disorder
that's pretty sad condition you have there, I hope what I am going to provide will be much useful for you.


In order to prevent the user from installing new apps from Play Store, you may try any of this method from this article: 


https://useboomerang.com/2018/07/25/3-ways-block-kids-installing-apps-android-phone/

Regarding with allowing only specific websites to access, you can try from any of the followings:


https://www.lifewire.com/how-to-block-websites-android-4179021
https://techwiser.com/block-websites-on-android/

I am not really sure if we are able to set this up, however, try this:


https://superuser.com/questions/1156080/how-to-stop-users-from-viewing-saved-passwords-in-chrome



Maybe the link below can be a good addition:
- https://www.howtogeek.com/223530/how-to-lock-down-your-android-tablet-or-smartphone-for-kids/