WhatsApp is owned by FaceBook, so I'd have reservations about using it anyway. One family member insist s on using it so I coperate with her for the odd message.
They claim end-to-end encryption using Open Whisper Systems technology but I ain't so sure as they use public networks to transport the messages. Others, such as Apple's Message app for iOS and macOS, supply in-device encrytption / decryption but as a further security feature use their internal private network for the transport infrastructure.
Have you queried WhatsApp/TodayFM/Electric Picnic?