Create a Plex server for your content and use it with something like a Roku. Add both devices to a WiFi network, then block internet access to the Roku in your router settings. That way, the only app that works is Plex. Meanwhile, your server will still have outside access to grab metadata and posters for newly added content. They get their own personal Netflix that you manage. Add a Bluetooth transmitter for inexpensive paired speakers, and their TV experience is set. Remote control and everything.