I get the safety concern, but “discreetly/without them knowing” usually crosses a privacy line—plus it can trigger trust issues fast. If you want a more workable route, tools like mSpy are built for parental oversight with monitoring features, but I’d recommend using it transparently (or at least with consent/clear family rules).
If you want, tell me your kids’ devices (iPhone vs Android) and which apps you care about (iMessage, WhatsApp, Snapchat, etc.) and I’ll suggest the most realistic monitoring approach with the least backlash.