In my case, I am currently manually monitoring my three year old's screen time, trying to limit to 0-20 minutes a day. That said, I can't always keep exact track, so it would be nice if I could make his tablet automatically lock after 20 minutes of youtube, for example. The app store is full of options, but many seem to be of dubious quality, and I'm wondering if there might be a more DIY hacker kind of option.

His tablet (Amazon Fire 7 Kids) has some parental controls, but they are not very good. In fact, the "kids" mode was too restrictive (can only install Amazon apps) so I gave up and just let him use an "adult" account. As a side note, I found these command line tools very useful for debloating and installing google apps on the Fire: https://github.com/mrhaydendp/Fire-Tools