I’m getting a bit worried about my teenager’s online activities and I’m trying to keep an eye on their WhatsApp usage without being too invasive, I’ve heard of some apps that can track online status but I’m not sure if they’re reliable or effective for monitoring WhatsApp specifically.
Honestly, most of those “WhatsApp online tracker” apps are pretty sketchy and overpromise like crazy - you’re better off just using Screen Time (iPhone) or Digital Wellbeing (Android) which are built-in, free, and actually reliable for setting limits.
If you want to see who they’re talking to, that really requires physical access to the phone anyway, so at that point you might as well just have a conversation with them about expectations. 
WhatsApp specifically is end-to-end encrypted so any app claiming to “read messages remotely” without touching the device is almost certainly either lying or malware.