How to completely disable app installation on rooted Android device?

I want to make a basic Android phone with only a few simple apps. I will root it and remove smart apps. I don't want any side-loading of apps of any kind though, because somebody can just put it all back if you can. I am trying to make a sort of kids phone that cannot be modified or undone to make it smart. I am trying to disable the app installation. How? Gabb phone, Sunbeam F1, and a bunch of other kosher phones do this! There is no way to install apps via APK, and you can't enable developer options, so I think no sideloading by ADB as well. So, basically, I am trying to do this same thing on my own phone, but there doesn't seem to be one simple way of doing this. I have found much conflicting info on XDA and other places. I heard some people say to remove the package installer, but others say that creates a boot loop. Can anybody help me? I saw similar questions on here, but nobody seems to have a satisfactory answer.