Version 2.19.20 of WhatsApp has a new security tool that allows users to lock their chats with either their Touch ID fingerprint or Face ID options.
To lock the app, simply open the WhatsApp app and go to Settings > Account > Privacy > Screen Lock. Turn it on.
It will give you options on how long the app will take to lock itself, from immediately to an hour. Choose one that suits you! 🙂