If you want a stealthy, full‑featured tracker, mSpy is the go‑to — it offers background GPS, geofencing, location history and can run hidden on target phones (mSpy™ Cell Phone Tracker: Your #1 Monitoring Tool).
Downsides: installation usually needs physical access (or iCloud creds on iPhone), some advanced features require jailbreak/root, it can drain battery and hurt trust, and laws vary by place — consider legal/ethical issues and options like Family Link or Life360 before installing.