I was thinking something like an esp32 + mesthastic / LoRa + REST API on the LAN, discoverable via multi cast. The "landline" is a tablet or phone with an app that talks to the esp32. Separately, a parent with the app does the Diffie-Hellman key exchange over SMS, NFC, or some other channel with the friends who also have the app, and you know their identity. The phone app updates the device with the friend's keys, they do the same thing on their end, and voila you're in business. The kids can talk securely, you can read that the kids say via the LAN, no goddamned third parties.