Hey Haley25, welcome to the forum! 
Unfortunately, Apple’s Find My is designed to notify the device owner when it’s added to an account, so doing it secretly through that app is really tricky. What worked for us was using mSpy instead — it runs quietly in the background and gives you way more than just location, like texts and app activity. It’s been a game-changer for keeping tabs on my teens without a big confrontation every time!