How much does Chatwatch cost?
The app costs $2 per week to keep track on a single pair of friends - but you can pay more to monitor up to 10 people at a time. Chatwatch was available from both the App Store and Google Play, but it appears to have been pulled from both already.
How WhatsApp last seen app works?
Last seen and online tell you the last time your contacts used WhatsApp, or if they're online. If a contact is online, they have WhatsApp open in the foreground on their device and are connected to the Internet.