We want to use any of our Android phones or tablets (masters) to control a single-purpose Android device that is tucked away in a closet (slave). The master devices should have full control over/unlimited access to the slave–starting/stopping apps, adjusting volume, looking at wi-fi settings, etc.
The devices can communicate via wi-fi network or Bluetooth, whichever has better throughput and range.
Security-wise, nothing needs to be hidden. In fact, the easier it is to see that it’s running, the better. I’m also not particularly security-concious about the slave. It will have no SIM card and it’s only network connection will be to the home wi-fi network to reach the Internet. It’s in our home, which is about 600 heavily wooded feet from our two neighbours. Since the app will be running on several master devices that we take into town, it’s important that I’m not introducing a vulnerability there.